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java 代码片段:)

句法

访问修饰符：

公共
定义为公共的类、方法或变量可以被任何类或方法访问。

受保护
protec++ted 可以被同一个包的类访问，也可以被该类的子类访问，也可以在同一个类内访问。
（注意： 该关键字仅适用于嵌套类）

私人
定义为 private 的私有类、方法或变量只能在类内部访问。

默认
默认值只能在包内访问。默认情况下，所有类、方法和变量都具有默认范围。

package com.example;

public class example {
    private int privatevar = 10;
    int defaultvar = 20;
    protected int protectedvar = 30;
    public int publicvar = 40;

    public static void main(string[] args) {
        example example = new example();

        // accessing all variables within the same class
        system.out.println("private var: " + example.privatevar);
        system.out.println("default var: " + example.defaultvar);
        system.out.println("protected var: " + example.protectedvar);
        system.out.println("public var: " + example.publicvar);
    }
}

//another file
package com.example;

public class example1 {
    public static void main(string[] args) {
        example example = new example();

        // private variable is inaccessible
        system.out.println("default var: " + example.defaultvar);
        system.out.println("protected var: " + example.protectedvar);
        system.out.println("public var: " + example.publicvar);
    }
}

// file: different.java
package com.example.subpackage;

import com.example.example;

public class different extends example {
    public static void main(string[] args) {
        example example = new example();
        different subclassexample = new different();

        // private variable is inaccessible
        // default variable is inaccessible
        // protected variable is inaccessible
        system.out.println("public var: " + example.publicvar);

        // accessing protected variable through inheritance
        system.out.println("protected var (through inheritance): " + subclassexample.protectedvar);
    }
}

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数据类型

public class main {

    // class to demonstrate structure equivalent
    static class person {
        string name; // default value is null
        int age; // default value is 0
        float salary; // default value is 0.0f
    }

    // enumeration to demonstrate enum equivalent
    enum color { red, green, blue }

    public static void main(string[] args) {

        // basic data types
        int a = 10; // default value is 0
        float b = 5.5f; // default value is 0.0f
        char c = 'a'; // default value is '\u0000'
        double d = 2.3; // default value is 0.0d
        long e = 123456789l; // default value is 0l
        short f = 32000; // default value is 0
        byte g = 100; // default value is 0

        // array
        int[] arr = {1, 2, 3, 4, 5};

        // structure equivalent
        person person1 = new person();
        person1.age = 30;
        person1.salary = 55000.50f;

        // enumeration
        color mycolor = color.red;
    }
}

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超载

当同一个类中多个方法具有相同名称但参数不同时，就会发生方法重载。这是在编译时解决的。

class mathoperations {
    // method to add two integers
    public int add(int a, int b) {
        return a + b;
    }

    // method to add three integers
    public int add(int a, int b, int c) {
        return a + b + c;
    }

    // method to add two double values
    public double add(double a, double b) {
        return a + b;
    }
}

public class main {
    public static void main(string[] args) {
        mathoperations math = new mathoperations();

        system.out.println(math.add(2, 3));
        system.out.println(math.add(1, 2, 3));
        system.out.println(math.add(2.5, 3.5));
    }
}

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压倒一切

当子类为其超类中已定义的方法提供特定实现时，就会发生方法重写。这在运行时解决了

class animal {
    public void makesound() {
        system.out.println("animal makes a sound");
    }
}

class dog extends animal {
    @override
    public void makesound() {
        system.out.println("dog barks");
    }
}

public class main {
    public static void main(string[] args) {
        animal myanimal = new animal();
        myanimal.makesound(); // calls method in animal class

        dog mydog = new dog();
        mydog.makesound(); // calls overridden method in dog class
    }
}

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大批

数组和对象始终通过引用传递，而不是复制。原始数据类型按值传递。

public class main {
    public static void main(string[] args) {

        // integer array
        int[] intarray = new int[5]; // default values: 0
        int[] intarray2 = {1, 2, 3, 4, 5}; // initialized with specific values

        // float array
        float[] floatarray = new float[5]; // default values: 0.0f
        float[] floatarray2 = {1.1f, 2.2f, 3.3f, 4.4f, 5.5f}; // initialized with specific values

    }
}

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默认关键字

默认方法是在接口中使用 default 关键字定义的方法。这允许接口提供方法实现，而不影响实现该接口的类。

interface animal {
    void makesound(); // abstract method

    // default method
    default void sleep() {
        system.out.println("sleeping...");
    }
}

class dog implements animal {
    public void makesound() {
        system.out.println("bark");
    }
}

public class main {
    public static void main(string[] args) {
        animal dog = new dog();

        dog.makesound(); // output: bark
        dog.sleep();     // output: sleeping...

    }
}

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静态关键字

它表示成员（变量、方法或嵌套类）属于类本身，而不是属于类的实例。
（注意： 我们不能在非静态方法中声明静态变量）

public class staticexample {
    // static variable
    static int staticvariable = 10;

    // instance variable
    int instancevariable = 20;

    // static method
    static void staticmethod() {
        // can access static variable
        system.out.println(staticvariable);

        // cannot access instance variable directly
        // system.out.println(instancevariable);
    }

    // instance method
    void instancemethod() {

        // can access static variable
        system.out.println(staticvariable);

        // can access instance variable
        system.out.println(instancevariable);
    }

    // static nested class
    static class staticnestedclass {
        void display() {
            system.out.println("static nested class method called.");
            // can access static members of the outer class
            system.out.println("static variable from nested class: " + staticvariable);
        }
    }

    public static void main(string[] args) {
        // call static method
        staticexample.staticmethod();

        // create an instance of the class
        staticexample example = new staticexample();
        // call instance method
        example.instancemethod();

        // create an instance of the static nested class
        staticnestedclass nestedclass = new staticnestedclass();
        nestedclass.display();
    }
}

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初始化

（注意： 只有实例变量和静态变量会被初始化为默认值）

import java.util.scanner;

public class inputexample {
    public static void main(string[] args) {
          int num;
          int num1=0;
          system.out.println(num); //throws error
          system.out.println(num1); // prints 0 
    }
}

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显示类的方法及其参数

import java.lang.reflect.method;
import java.lang.reflect.parameter;
import java.util.linkedlist;

public class main {
    public static void main(string[] args) {
        class<?> clazz = linkedlist.class;
        // get all methods of the class
        method[] methods = clazz.getdeclaredmethods();
        // print the names and parameters of all methods
        for (method method : methods) {
            system.out.print("method name: " + method.getname());
            system.out.print(", parameters: ");
            parameter[] parameters = method.getparameters();
            for (parameter parameter : parameters) {
                system.out.print(parameter.gettype().getname() + " " + parameter.getname() + ", ");
            }
            system.out.println();
        }
    }
}

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基本输入/输出

import java.util.scanner;

public class inputexample {
    public static void main(string[] args) {
        scanner scan = new scanner(system.in);

        // reading integer input
        system.out.print("enter an integer: ");
        int intvalue = scan.nextint();
        system.out.println("integer entered: " + intvalue);

        // reading double input
        system.out.print("enter a double: ");
        double doublevalue = scan.nextdouble();
        system.out.println("double entered: " + doublevalue);

        // reading string input (including spaces)
        system.out.print("enter a string: ");
        scan.nextline(); // consume the newline character
        string stringvalue = scan.nextline();
        system.out.println("string entered: " + stringvalue);

        // reading character input
        system.out.print("enter a character: ");
        char charvalue = scan.next().charat(0);
        system.out.println("character entered: " + charvalue);

        // reading boolean input
        system.out.print("enter a boolean (true/false): ");
        boolean booleanvalue = scan.nextboolean();
        system.out.println("boolean entered: " + booleanvalue);

        // reading input until there is no more input available
        system.out.println("enter multiple lines of input (ctrl+d / ctrl+z to exit):");
        scan.nextline(); // consume the newline character
        while (scan.hasnext()) {
            string line = scan.nextline();
            system.out.println("input: " + line);
        }

        scan.close();
    }
}

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for循环

public class main {
    public static void main(string[] args) {
        for (int i = 0; i < 10; ++i) {
            system.out.print(i + " ");
        }
        system.out.println();
    }
}

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while 循环

public class main {
    public static void main(string[] args) {
        int i = 0;
        while (i < 10) {
            system.out.print(i + " ");
            ++i;
        }
        system.out.println();
    }
}

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功能

public class main {
    public static int add(int a, int b) {
        return a + b;
    }

    public static void main(string[] args) {
        int result = add(5, 3);
        system.out.println("sum: " + result);
    }
}

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类和对象

public class rectangle {
    private int width, height;

    public rectangle(int w, int h) {
        this.width = w;
        this.height = h;
    }

    public int area() {
        return width * height;
    }

    public static void main(string[] args) {
        rectangle rect = new rectangle(5, 3);
        system.out.println("area: " + rect.area());
    }
}

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遗产

public class shape {
    public void draw() {
        system.out.println("drawing a shape");
    }
}

public class circle extends shape {
    @override
    public void draw() {
        system.out.println("drawing a circle");
    }

    public static void main(string[] args) {
        shape shape = new circle();
        shape.draw();
    }
}

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最终关键词

它是通过添加final关键字

创建的

1) 最终变量
该变量一旦初始化就无法更改

public class finalvariableexample {
    public static void main(string[] args) {
        final int x = 10;
        // x = 20; // this will cause a compilation error
        system.out.println(x);// 10
    }
}

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2）最终方法
这些方法不能被覆盖

class parent {
    final void show() {
        system.out.println("this is a final method.");
    }
}

class child extends parent {
    // void show() { // this will cause a compilation error
    //    system.out.println("trying to override.");
    // }
}

public class finalmethodexample {
    public static void main(string[] args) {
        child c = new child();
        c.show(); // output: this is a final method.
    }
}

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超级关键词

java中的super关键字用于引用当前对象的直接父类。它提供了一种从子类中访问父类的成员（方法和变量）的方法。

class parent {
    int x = 10;

    void display() {
        system.out.println("parent's display method");
    }

    parent() {
        system.out.println("parent's constructor");
    }
}

class child extends parent {
    int x = 20;

    void display() {
        super.display(); // calls the display method of parent class
        system.out.println("child's display method");
    }

    child() {
        super(); // calls the constructor of parent class
        system.out.println("child's constructor");
    }
}

public class test {
    public static void main(string[] args) {
        child obj = new child();
        system.out.println("value of x in child: " + obj.x);
        obj.display();
    }
}

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抽象的

java中的abstract关键字用于定义抽象类和抽象方法。抽象类不能直接实例化，只能进行子类化。抽象方法没有主体，必须由子类实现。
它包含抽象方法和具体方法

abstract class animal {
    // abstract method (does not have a body)
    abstract void makesound();

    // regular method
    void eat() {
        system.out.println("this animal is eating.");
    }
}

class dog extends animal {
    // the body of the abstract method is provided here
    void makesound() {
        system.out.println("woof");
    }
}

public class main {
    public static void main(string[] args) {
        dog mydog = new dog();
        mydog.makesound();  // output: woof
        mydog.eat();        // output: this animal is eating.
    }
}

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当一个接口想要继承另一个接口的方法时，可以使用extends关键字。

抽象类可以实现接口。这意味着抽象类使用implements关键字提供了接口的部分实现。

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一个抽象类可以使用 extends 关键字扩展另一个抽象类

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界面

是类似于类的引用类型，只包含常量、默认方法、静态方法和方法签名
(注意： 接口中只存在默认方法的定义)
（注：接口内声明的常量为public、static和final）

interface animal {
    void makesound(); // abstract method
}

class dog implements animal {
    public void makesound() {
        system.out.println("bark");
    }
}

class cat implements animal {
    public void makesound() {
        system.out.println("meow");
    }
}

public class main {
    public static void main(string[] args) {
        animal dog = new dog();
        animal cat = new cat();

        dog.makesound(); // output: bark
        cat.makesound(); // output: meow
    }
}

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克服钻石问题（多重继承）：

interface a {
    void display();
}

interface b extends a {
    default void display() {
        system.out.println("display from b");
    }
}

interface c extends a {
    default void display() {
        system.out.println("display from c");
    }
}

class d implements b, c {

    @override
    public void display() {
        b.super.display();
        c.super.display();
        system.out.println("display from d");
    }
}

public class main {
    public static void main(string[] args) {
        d d = new d();
        d.display();

        //output
        //display from b
        //display from c
        //display from d
    }
}

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列表

import java.util.linkedlist;
import java.util.list;
import java.util.arraylist;

public class listexample {
    public static void main(string[] args) {

        // declare list as linkedlist
        list<character> list = new linkedlist<>();

        // add elements
        list.add('a');
        list.add('b');
        list.add('c');

        // change implementation to arraylist
        list = new arraylist<>(list);

        // add more elements
        list.add('d');
        list.add('e');

        // change implementation to vector
        list = new vector<>(list);

        // add more elements
        list.add('f');
        list.add('g');
    }
}

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队列

java 中的 queue 接口是 java 集合框架的一部分，代表一个设计用于在处理之前保存元素的集合。它通常以 fifo（先进先出）方式对元素进行排序，但其他排序也是可能的，例如 deque 实现中的 lifo（后进先出）。

import java.util.linkedlist;
import java.util.priorityqueue;
import java.util.arraydeque;
import java.util.queue;
import java.util.deque;

public class queueexample {
    public static void main(string[] args) {
        // using linkedlist as a queue
        queue<string> queue = new linkedlist<>();
        queue.add("java");
        queue.add("python");
        queue.add("c++");

        // reassigning to priorityqueue
        queue = new priorityqueue<>();
        queue.add("java");
        queue.add("python");
        queue.add("c++");

        // reassigning to arraydeque
        queue = new arraydeque<>();
        queue.add("java");
        queue.add("python");
        queue.add("c++");
    }
}

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数组列表

1.动态调整大小
2.实现列表接口

import java.util.arraylist;

public class main {
    public static void main(string[] args) {
        arraylist<integer> list = new arraylist<>();
        list.add(1);
        list.add(2);
        list.add(3);

        for (int num : list) {
            system.out.print(num + " ");
        }
        system.out.println();

        // inserting an element
        list.add(2, "ruby");

        // accessing elements
        system.out.println("element at index 3: + list.get(3));

        // removing an element 
        list.remove(1);

        // size of the list 
        system.out.println(list.size());

        // check if list is empty 
        system.out.println("is list empty? + list.isempty());

        // check if list contains an element 
        system.out.println(list.contains("java"));

        // index of an element 
        system.out.println(list.indexof("javascript"));

        // last index of an element 
        system.out.println(list.lastindexof("ruby"));

        // clear the list
        list.clear()
    }
}

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堆

import java.util.stack;

public class stackexample {
    public static void main(string[] args) {

        // create a stack
        stack<string> stack = new stack<>();

        // push elements onto the stack
        stack.push("java");
        stack.push("python");
        stack.push("c++");

        // print the stack after pushes
        system.out.println("stack after pushes: " + stack);

        // get the size of the stack
        int size = stack.size();
        system.out.println("size of stack: " + size);

        // peek at the top element without removing it
        string topelement = stack.peek();
        system.out.println("top element (peek): " + topelement);

        // pop an element from the stack
        string poppedelement = stack.pop();
        system.out.println("popped element: " + poppedelement);

        // print the stack after pop
        system.out.println("stack after pop: " + stack);

        // check if the stack is empty
        boolean isempty = stack.isempty();
        system.out.println("is stack empty? " + isempty);

        // get the size of the stack after pop
        size = stack.size();
        system.out.println("size of stack after pop: " + size);

        // search for an element
        int position = stack.search("java");
        system.out.println("position of element 'java': " + position);
    }
}

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链表

import java.util.linkedlist;

public class linkedlistexample {
    public static void main(string[] args) {
        // create a new linkedlist
        linkedlist<string> linkedlist = new linkedlist<>();

        // add elements to the linkedlist
        linkedlist.add("java");
        linkedlist.add("python");
        linkedlist.add("c++");
        system.out.println("initial linkedlist: " + linkedlist);

        // add element at specific index
        linkedlist.add(1, "javascript");
        system.out.println("after add(1, 'javascript'): " + linkedlist);

        // add element at the beginning
        linkedlist.addfirst("html");
        system.out.println("after addfirst('html'): " + linkedlist);

        // add element at the end
        linkedlist.addlast("css");
        system.out.println("after addlast('css'): " + linkedlist);

        // get elements
        system.out.println("first element: " + linkedlist.getfirst());
        system.out.println("last element: " + linkedlist.getlast());
        system.out.println("element at index 2: " + linkedlist.get(2));

        // remove elements
        linkedlist.remove(); // removes the first element
        system.out.println("after remove(): " + linkedlist);

        linkedlist.remove(2); // removes the element at index 2
        system.out.println("after remove(2): " + linkedlist);

        linkedlist.removefirst(); // removes the first element
        system.out.println("after removefirst(): " + linkedlist);

        linkedlist.removelast(); // removes the last element
        system.out.println("after removelast(): " + linkedlist);

        // check if the list contains a specific element
        system.out.println("contains 'python': " + linkedlist.contains("python"));

        // get the size of the list
        system.out.println("size of linkedlist: " + linkedlist.size());

        // clear the list
        linkedlist.clear();
        system.out.println("after clear(): " + linkedlist);

        // check if the list is empty
        system.out.println("is linkedlist empty? " + linkedlist.isempty());
    }
}

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哈希映射

import java.util.hashmap;

public class main {
    public static void main(string[] args) {
        hashmap<string, integer> map = new hashmap<>();
        map.put("alice", 90);
        map.put("bob", 85);

        for (string key : map.keyset()) {
            system.out.println(key + ": " + map.get(key));
        }
    }
}

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细绳

public class stringexamples {
    public static void main(string[] args) {
        // creating an empty string
        string emptystring = new string();
        system.out.println("empty string: \"" + emptystring + "\"");

        // creating a string from another string
        string original = "hello, world!";
        string copy = new string(original);
        system.out.println("copy of original: " + copy);

        // creating a string from a character array
        char[] chararray = {'j', 'a', 'v', 'a'};
        string fromchararray = new string(chararray);
        system.out.println("string from char array: " + fromchararray);

        // length of the string
        int length = original.length();
        system.out.println("length of original string: " + length);

        //check if two strings are equal
        string str1 = "java";
        string str2 = "java";

        system.out.println(str1.equals(str2)); // true


        // check if the string is empty
        boolean isempty = original.isempty();
        system.out.println("is original string empty? " + isempty);

        // character at a specific index
        char charat2 = original.charat(2);
        system.out.println("character at index 2 in original string: " + charat2);

        // convert to uppercase
        string uppercasestr = original.touppercase();
        system.out.println("uppercase: " + uppercasestr);

        // convert to lowercase
        string lowercasestr = original.tolowercase();
        system.out.println("lowercase: " + lowercasestr);

        // character methods
        char ch1 = 'a';
        char ch2 = 'a';
        char ch3 = '1';
        char ch4 = ' ';
        char ch5 = '\u2603'; // unicode character for snowman

        system.out.println("isdigit('1'): " + character.isdigit(ch3)); // true
        system.out.println("isletter('a'): " + character.isletter(ch1)); // true
        system.out.println("isletterordigit('a'): " + character.isletterordigit(ch2)); // true
        system.out.println("islowercase('a'): " + character.islowercase(ch2)); // true
        system.out.println("isuppercase('a'): " + character.isuppercase(ch1)); // true
        system.out.println("iswhitespace(' '): " + character.iswhitespace(ch4)); // true
        system.out.println("tolowercase('a'): " + character.tolowercase(ch1)); // 'a'
        system.out.println("touppercase('a'): " + character.touppercase(ch2)); // 'a'

        // reverse a string using stringbuilder
        string reversedstr = reverseusingstringbuilder(original);
        system.out.println("reversed string: " + reversedstr);
    }

    public static string reverseusingstringbuilder(string str) {
        stringbuilder stringbuilder = new stringbuilder(str);
        stringbuilder.reverse();
        return stringbuilder.tostring();
    }
}

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文件输入/输出

import java.io.file;
import java.io.filewriter;
import java.io.filereader;
import java.io.bufferedreader;
import java.io.ioexception;

public class main {
    public static void main(string[] args) {
        // write to file
        try (filewriter writer = new filewriter("example.txt")) {
            writer.write("hello, file i/o!\n");
        } catch (ioexception e) {
            e.printstacktrace();
        }

        // read from file
        try (bufferedreader reader = new bufferedreader(new filereader("example.txt"))) {
            string line;
            while ((line = reader.readline()) != null) {
                system.out.println(line);
            }
        } catch (ioexception e) {
            e.printstacktrace();
        }
    }
}

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异常处理

（注意： 捕获异常时，更具体的异常必须列在更一般的异常之前）

public class main {
    public static int divide(int a, int b) {
        if (b == 0) {
            throw new illegalargumentexception("division by zero");
        }
        return a / b;
    }

    public static void main(string[] args) {
        try {
            int result = divide(10, 0);
            system.out.println("result: " + result);
        } catch (illegalargumentexception e) {
            system.out.println("error: " + e.getmessage());
        }
    }
}

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内部类

内部类是在另一个类中定义的类。

会员内部类成员内部类与外部类的实例相关联。它可以访问外部类的实例变量和方法。

public class outerclass {
    private string outerfield = "outer field";

    class innerclass {
        void display() {
            system.out.println("accessing outer field: " + outerfield);
        }
    }

    public static void main(string[] args) {
        outerclass outer = new outerclass();
        outerclass.innerclass inner = outer.new innerclass();
        inner.display();
    }
}

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静态嵌套类静态嵌套类是在另一个类中定义的静态类。它无权访问外部类的实例成员，但可以访问其静态成员。

public class outerclass {
    private static string staticfield = "static field";

    static class staticnestedclass {
        void display() {
            system.out.println("accessing static field: " + staticfield);
        }
    }

    public static void main(string[] args) {
        outerclass.staticnestedclass nested = new outerclass.staticnestedclass();
        nested.display();
    }
}

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本地内部类局部内部类在块或方法内定义，并且可以访问局部变量（必须是最终的或实际上最终的）和外部类的成员。

public class outerclass {
    void method() {
        final string localvariable = "local variable";

        class localinnerclass {
            void display() {
                system.out.println("accessing local variable: " + localvariable);
            }
        }

        localinnerclass localinner = new localinnerclass();
        localinner.display();
    }

    public static void main(string[] args) {
        outerclass outer = new outerclass();
        outer.method();
    }
}

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匿名内部类匿名内部类是一种没有名称的内部类。它可以精确地扩展一个类或精确地实现一个接口

class baseclass {
    void display() {
        system.out.println("baseclass display()");
    }
}

public class main {
    public static void main(string[] args) {
        baseclass obj = new baseclass() {
            @override
            void display() {
                system.out.println("anonymous inner class display()");
            }
        };

        obj.display();
    }
}

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拉姆达表达式

import java.util.arraylist;
import java.util.list;

public class main {
    public static void main(string[] args) {
        list<integer> nums = new arraylist<>();
        nums.add(1);
        nums.add(2);
        nums.add(3);
        nums.add(4);
        nums.add(5);

        nums.replaceall(n -> n * 2);

        for (int num : nums) {
            system.out.print(num + " ");
        }
        system.out.println();
    }
}

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switch 语句

public class main {
    public static void main(string[] args) {
        int day = 3;
        switch (day) {
            case 1:
                system.out.println("monday");
                break;
            case 2:
                system.out.println("tuesday");
                break;
            case 3:
                system.out.println("wednesday");
                break;
            case 4:
                system.out.println("thursday");
                break;
            case 5:
                system.out.println("friday");
                break;
            case 6:
                system.out.println("saturday");
                break;
            case 7:
                system.out.println("sunday");
                break;
            default:
                system.out.println("invalid day");
                break;
        }
    }
}

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普通指针

java 不像 c++ 那样使用显式指针；

小写和大写：

public class Main {
    public static void main(String[] args) {
        String str1 = "Hello, World!";
        String str2= "HELLO, WORLD!";

        String lowerStr = str2.toLowerCase();
        String upperStr = str1.toUpperCase();

        System.out.println(lowerStr);  // Output: hello, world!
        System.out.println(upperStr);  // Output: HELLO, WORLD!
    }
}

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