学java要学多久才能工作 Java学习周期和就业时间预估

// 多态示例
public class Shape {
    public void draw() {
        System.out.println("Drawing a shape");
    }
}
<p>public class Circle extends Shape {
@Override
public void draw() {
System.out.println("Drawing a circle");
}
}</p><p>public class Rectangle extends Shape {
@Override
public void draw() {
System.out.println("Drawing a rectangle");
}
}</p><p>public class Main {
public static void main(String[] args) {
Shape shape1 = new Circle();
Shape shape2 = new Rectangle();</p><pre class='brush:java;toolbar:false;'>    shape1.draw(); // 输出: Drawing a circle
    shape2.draw(); // 输出: Drawing a rectangle
}

// 性能优化示例
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
<p>public class PerformanceTest {
public static void main(String[] args) {
int size = 1000000;
List<Integer> arrayList = new ArrayList<>();
List<Integer> linkedList = new LinkedList<>();</p><pre class='brush:java;toolbar:false;'>    // 测试ArrayList的性能
    long startTime = System.nanoTime();
    for (int i = 0; i < size; i++) {
        arrayList.add(i);
    }
    long endTime = System.nanoTime();
    System.out.println("ArrayList添加时间: " + (endTime - startTime) + " ns");

    // 测试LinkedList的性能
    startTime = System.nanoTime();
    for (int i = 0; i < size; i++) {
        linkedList.add(i);
    }
    endTime = System.nanoTime();
    System.out.println("LinkedList添加时间: " + (endTime - startTime) + " ns");
}