集合、排序和实用方法中的部分泛型

list<string> strings = new arraylist<>();
strings.add("hello");
// adding a non-string value will now cause a compile-time error.

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

public class genericlistexample {
    public static void main(string[] args) {
        list<string> names = new arraylist<>();
        names.add("alice");
        names.add("bob");

        // the following line would cause a compile-time error:
        // names.add(123); // error: cannot add integer to list<string>

        for (string name : names) {
            system.out.println(name);
        }
    }
}

import java.util.hashset;
import java.util.set;

public class genericsetexample {
    public static void main(string[] args) {
        set<integer> numbers = new hashset<>();
        numbers.add(10);
        numbers.add(20);
        numbers.add(30);

        // compile-time error if a non-integer is added:
        // numbers.add("forty"); // error

        for (integer num : numbers) {
            system.out.println(num);
        }
    }
}

import java.util.hashmap;
import java.util.map;

public class genericmapexample {
    public static void main(string[] args) {
        map<string, integer> phonebook = new hashmap<>();
        phonebook.put("alice", 12345);
        phonebook.put("bob", 67890);

        // the following would cause a compile-time error:
        // phonebook.put(123, "charlie"); // error

        for (map.entry<string, integer> entry : phonebook.entryset()) {
            system.out.println(entry.getkey() + ": " + entry.getvalue());
        }
    }
}

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

public class genericsortingexample {
    public static void main(string[] args) {
        list<integer> numbers = new arraylist<>();
        numbers.add(5);
        numbers.add(3);
        numbers.add(8);
        numbers.add(1);

        collections.sort(numbers); // sorts in natural order
        system.out.println("sorted numbers: " + numbers);
    }
}

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

public class utilitymethodsexample {
    public static void main(string[] args) {
        list<string> items = new arraylist<>();
        items.add("apple");
        items.add("banana");
        items.add("cherry");

        // shuffle the list
        collections.shuffle(items);
        system.out.println("shuffled list: " + items);

        // reverse the list
        collections.reverse(items);
        system.out.println("reversed list: " + items);

        // frequency of an item
        int freq = collections.frequency(items, "apple");
        system.out.println("frequency of apple: " + freq);
    }
}

  List list = new ArrayList(); // Raw type
  list.add("String");
  list.add(123); // No compile-time error, but may cause runtime issues