Java 值傳遞 與 引用傳遞，形參/實參

java中方法參數傳遞方式是按值傳遞。

• 如果參數是基本類型，傳遞的是基本類型的【字面量值的拷貝】。

• 如果參數是引用類型，傳遞的是該參量所引用的對象在堆中【地址值的拷貝】。

參考：

Java 值傳遞 or 引用傳遞？

Java 到底是值傳遞還是引用傳遞？

測試一：基本類型作爲參數傳遞

/**
 * 基本類型作爲參數傳遞
 */
public class Test1 {

    public static void main(String[] args) {
        //基本類型作爲參數傳遞
        int num = 2;
        System.out.println("before change , num = " + num);
        changeData(num);
        System.out.println("after change , num = " + num);
    }

    public static void changeData(int num) {
        num = 10;
    }

}
//before change , num = 2
//after change , num = 2

測試二：對象作爲參數傳遞

/**
 * 對象作爲參數傳遞
 */
public class Test2 {

    public static void main(String[] args) {
        //對象作爲參數傳遞
        A a = new A("hello");
        System.out.println("before change , a.str = " + a.str);
        changeData(a);
        System.out.println("after change , a.str = " + a.str);
    }

    public static void changeData(A a) {
        a.str = "hi";
    }

    static class A {
        public String str;

        public A(String str) {
            this.str = str;
        }
    }

}
//before change , a.str = hello
//after change , a.str = hi

測試三

import java.util.Arrays;

public class Test3 {

    @Override
    public String toString() {
        return super.toString();
    }

    public static void main(String[] args) {
        String str = new String("987");
        StringBuilder stringBuilder = new StringBuilder("987");
        char[] ch1 = new char[]{'1', '2', '3'};
        char[] ch2 = new char[]{'1', '2', '3'};

        change(str, stringBuilder, ch1, ch2);

        System.out.println(str);
        System.out.println(stringBuilder);
        System.out.println(ch1);
        System.out.println(ch2);
    }

    public static void change(String str, StringBuilder stringBuilder,
                              char[] ch1, char[] ch2) {
        str = "987654";
        stringBuilder.append("5");
        ch1[0] = '0';
        ch2 = new char[]{'9', '8', '7'};
    }

}
//987
//9875
//023
//123

測試四：對象的引用傳遞

public class Point {
    private int x;
    private int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    public void setLocation(int x, int y) {
        this.x = x;
        this.y = y;
    }

    private static void modifyPoint(Point p1, Point p2) {
        //此處拷貝對象的地址值
        System.out.println("    modifyPoint內 交換前 p1============" + p1);
        System.out.println("    modifyPoint內 交換前 p2============" + p2);
        Point tmpPoint = p1;
        p1 = p2;
        p2 = tmpPoint;
        System.out.println("        modifyPoint內 交換後 p1============" + p1);
        System.out.println("        modifyPoint內 交換後 p2============" + p2);

        //修改原有對象
        p1.setLocation(6, 6);
        //新建對象
        p2 = new Point(5, 5);
        System.out.println("            modifyPoint內 結果 p1============" + p1);
        System.out.println("            modifyPoint內 結果 p2============" + p2);
    }

    public static void main(String[] args) {
        Point p1 = new Point(1, 1);
        Point p2 = new Point(2, 2);
        System.out.println("modifyPoint前 p1============" + p1);
        System.out.println("modifyPoint前 p2============" + p2);
        modifyPoint(p1, p2);
        System.out.println("modifyPoint後 p1============" + p1);
        System.out.println("modifyPoint後 p2============" + p2);
        System.out.println("[" + p1.x + "," + p1.y + "],[" + p2.x + "," + p2.y + "]");
    }

}
//modifyPoint前 p1============Point@133314b
//modifyPoint前 p2============Point@b1bc7ed
//    modifyPoint內 交換前 p1============Point@133314b
//    modifyPoint內 交換前 p2============Point@b1bc7ed
//        modifyPoint內 交換後 p1============Point@b1bc7ed
//        modifyPoint內 交換後 p2============Point@133314b
//            modifyPoint內 結果 p1============Point@b1bc7ed
//            modifyPoint內 結果 p2============Point@7cd84586
//modifyPoint後 p1============Point@133314b
//modifyPoint後 p2============Point@b1bc7ed
//[1,1],[6,6]

• modifyPoint前
  
• modifyPoint內 交換前

• modifyPoint內 交換後

• modifyPoint內 結果