Java 参数传递都是值传递

Java 参数传递都是值传递，验证代码如下

public class ParamTransferTest {

public static void swap(int a, int b) {
int temp = a;
a = b;
b = temp;
System.out.println("swap() a = " + a + " b = " + b);
}

public static void swap(DataWrap dataWrap) {
int temp = dataWrap.a;
dataWrap.a = dataWrap.b;
dataWrap.b = temp;

System.out.println("swap() datawrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);
}

public static void newObject(DataWrap dataWrap) {
dataWrap = new DataWrap();

System.out.println("newObject() datawrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);
}

public static void destoryObject(DataWrap dataWrap) {
dataWrap = null;

System.out.println("destoryObject() datawrap = " + dataWrap);
}

public static void swapObject(DataWrap a, DataWrap b){
System.out.println("in swap before a = " + a + " b = " + b);
DataWrap temp = a;
a = b;
b = temp;
System.out.println("in swap after a = " + a + " b = " + b);
}

public static void main(String[] args) {
// TODO Auto-generated method stub
//1.PrimitiveTransferTest
int a = 6;
int b = 9;

System.out.println("before swap, a = " + a + " b = " + b);
swap(a, b);
System.out.println("after swap, a = " + a + " b = " + b);

//2.RefTransferTest
DataWrap dataWrap = new DataWrap();
dataWrap.a = a;
dataWrap.b = b;

System.out.println("before swap, dataWrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);
swap(dataWrap);
System.out.println("after swap, dataWrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);

//3.in method create new object
System.out.println("before newObject, dataWrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);
newObject(dataWrap);
System.out.println("after newObject, dataWrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);

//4.in method destory object
System.out.println("before destoryObject, dataWrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);
destoryObject(dataWrap);
System.out.println("after destoryObject, dataWrap = " + dataWrap + " Field a = " + dataWrap.a + " Field b = " + dataWrap.b);

//5.swap object
DataWrap dataWrapA = new DataWrap();
DataWrap dataWrapB = new DataWrap();
System.out.println("before swap, dataWrapA: " + dataWrapA + " dataWrapB = = " + dataWrapB);
swapObject(dataWrapA, dataWrapB);
System.out.println("after swap, dataWrapA: " + dataWrapA + " dataWrapB: " +dataWrapB);
}
}

　　DataWrap.java 

public class DataWrap {
public int a;
public int b;
}

　　运行结果：

before swap, a = 6 b = 9
swap() a = 9 b = 6
after swap, a = 6 b = 9
before swap, dataWrap = DataWrap@659e0bfd Field a = 6 Field b = 9
swap() datawrap = DataWrap@659e0bfd Field a = 9 Field b = 6
after swap, dataWrap = DataWrap@659e0bfd Field a = 9 Field b = 6
before newObject, dataWrap = DataWrap@659e0bfd Field a = 9 Field b = 6
newObject() datawrap = DataWrap@2a139a55 Field a = 0 Field b = 0
after newObject, dataWrap = DataWrap@659e0bfd Field a = 9 Field b = 6
before destoryObject, dataWrap = DataWrap@659e0bfd Field a = 9 Field b = 6
destoryObject() datawrap = null
after destoryObject, dataWrap = DataWrap@659e0bfd Field a = 9 Field b = 6
before swap, dataWrapA: DataWrap@15db9742 dataWrapB = = DataWrap@6d06d69c
in swap before a = DataWrap@15db9742 b = DataWrap@6d06d69c
in swap after a = DataWrap@6d06d69c b = DataWrap@15db9742
after swap, dataWrapA: DataWrap@15db9742 dataWrapB: DataWrap@6d06d69c

　　如上，简单数据类型就不多说了。让人疑惑的往往是在参数是引用的时候。

由代码中验证的第3.4.5.步可以看出实际上参数类型是引用的时候，也是值传递。在函数内部栈会创建自己的指向原引用对象的引用，所以传引用的时候，修改对象本身，执行完函数后，对象本身确实发生了变化。而如果只是修改引用本身，并不会对传入的引用造成影响，因为在函数内部修改的引用只是局部的。

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