Java 參數傳遞都是值傳遞

Java 參數傳遞都是值傳遞，驗證代碼如下

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;
}

　　運行結果：

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，簡單數據類型就不多說了。讓人疑惑的往往是在參數是引用的時候。

由代碼中驗證的第3.4.5.步可以看出實際上參數類型是引用的時候，也是值傳遞。在函數內部棧會創建自己的指向原引用對象的引用，所以傳引用的時候，修改對象本身，執行完函數後，對象本身確實發生了變化。而如果只是修改引用本身，並不會對傳入的引用造成影響，因為在函數內部修改的引用只是局部的。

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