（原创）JAVA语言的参数传递

 JAVA语言的参数传递： 再一次感慨：java只有值传递 ，之所以有引用传递的假象是由于java中的对象，是通过引用操控的。下面是一个证明java参数是值传递而不是引用传递的一个例子：

public class Test {

	public static void exchangeString( String parameterStrA , String parameterStrB )
	{
        //交换前输出两个形参
        System.out.println( "in method exchangeString before exchange :" );
        System.out.println( "output parameterStrA@" + parameterStrA );
        System.out.println( "output parameterStrB@" + parameterStrB );
		
        //使用临时变量的方法交换两个形参
		String s = parameterStrA ;
		parameterStrA = parameterStrB;
		parameterStrB = s;
		
        //交换后输出两个形参
		System.out.println( "in method exchangeString after exchange :" );
		System.out.println( "output parameterStrA@" + parameterStrA );
		System.out.println( "output parameterStrB@" + parameterStrB );
		
	}
	
	public static void main( String[] args )
	{
        //初始化并输出两个实参
        String argumentStrA = new String( "i'm a" );
        String argumentStrB = new String( "i'm b" );
        System.out.println( "out of method before exchange:" );
		System.out.println( "output argumentStrA@" + argumentStrA );
		System.out.println( "output argumentStrB@" + argumentStrB );
		
        //调用交换方法
        exchangeString( argumentStrA , argumentStrB );
		
        //输出两个实参		
        System.out.println( "out of method after exchange:" );
        System.out.println( "output argumentStrA@" + argumentStrA );
        System.out.println( "output argumentStrB@" + argumentStrB );
		//***********************************************************************
		String s = new String("s");
		s = "ss";
		System.out.println( "s is "+s );
	}
}  

 

   下面为程序输出结果：                                                                                                                            

      out of method before exchange:
      output argumentStrA@i'm a
      output argumentStrB@i'm b
      in method exchangeString before exchange :
      output parameterStrA@i'm a
      output parameterStrB@i'm b
      in method exchangeString after exchange :
      output parameterStrA@i'm b
      output parameterStrB@i'm a
      out of method after exchange:
      output argumentStrA@i'm a
      output argumentStrB@i'm b    

 

为了进一步说明，再举一个例子

public class TestStringEvaluation{
 
    public static void main(String arg[]){
         
        String   string1=new String  ("string1 unchanged");
        MyString string2=new MyString("string2 unchanged");
        
        System.out.println(string1);
        System.out.println(string2.string);
        
        Change a=new Change();
         
        a.ChangeString(string1);
        a.ChangeMyString(string2);
         
        System.out.println(string1);
        System.out.println(string2.string);
    }
}
 
class Change{
    void ChangeString(String temp1){
        System.out.println( "before evaluation temp1.hashCode() is " + temp1.hashCode() );
    	temp1="string1 changed"; //赋值后形参temp1的值发生改变，变为“=”带来的新String对象的引用值。
    	System.out.println( "after evaluation temp1.hashCode() is " + temp1.hashCode() );
    }
    
    //形参temp2虽为实参的一个副本，但temp2.string却并非形参，而是实实在在的
    //MyString对象里的公有数据成员string，因此赋值后，temp2.string已经指向了一个新的对象
    //"="返回的新String对像，所以在后续的输出时有变化。
    //一定要注意  temp2 与  temp2.string的区别
    //一个是形参，一个是实际对象里的String对象引用
    
    void ChangeMyString(MyString temp2)
    {	System.out.println( "before evaluation temp2.string.hashCode() is " + temp2.string.hashCode() );
        temp2.string="string2 changed";
        System.out.println( "after evaluation temp2.string.hashCode() is " + temp2.string.hashCode() );
        
    }
}
 
class MyString
{
    public String string;
    MyString(String temp)
    {
        string=temp;
    }
} 

 

      输出结果是：                                                                                                                         
      string1 unchanged
      string2 unchanged
      before evaluation temp1.hashCode() is 1239571483
      after evaluation temp1.hashCode() is -513711916
      before evaluation temp2.string.hashCode() is -557379876
      after evaluation temp2.string.hashCode() is 1973800917
      string1 unchanged
      string2 changed