java对象引用在方法中被改变并理解结果

Question

以下程序输出

In second v.i:15
In first v.i:20

为什么这两种情况都不是 15。传递值对象，然后在第二个方法中更改对象引用。第二个方法应该是 15，并且在第一个方法中似乎也应该是 15

public class Test {
    /**
     * @param args
     */
    class Value{
        public int i = 15;
    }
    public static void main(String[] args) {
        Test t = new Test();
        t.first();
    }
    public void first(){
        Value v = new Value();
        v.i = 25;
        second(v);
        System.out.println("In First v.i:" + v.i);
    }
    public void second(Value v){
        v.i = 20;
        Value val = new Value();
        v = val;
        System.out.println("In second v.i:" + v.i);
    }
}

Answer 1

Java 方法实现是通过调用[引用，如果是对象，]值但不是完全是通过引用调用。

您正在传递一个对象Value v表示内联方法范围变量v引用在方法中创建的对象v 第一个()。这意味着对 v 引用的同一对象的任何修改也将反射(reflect)在调用端。但是，在您的第二方法中，您正在为Value创建一个新对象，但指向方法范围变量v。这个新对象的内存位置与传入方法参数的内存位置不同。要识别差异，请检查使用其引用变量创建的对象的 hashCode。

因此，在方法 second 中更改 v 的实例变量不会返回给该方法的调用者，除非该方法返回更改后的对象。您的方法在此处返回一个 void。

大多数时候，程序员会对调用者和被调用方法中使用的相同引用名称感到困惑。

请查看以下示例以了解其中的差异。我已经包含了第三个和第四个方法，以进一步解释它。

public class Test {
    class Value {
        int i = 15;
    }

    public void second( Value v ) {
        System.out.println( " 2.1.1: entered: v.i = " + v.i ); // 25
        System.out.println( " 2.1.2: v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );

        v = new Value();
        v.i = 9;
        System.out.println( " 2.2.1:   new V: v.i = " + v.i ); // 9
        System.out.println( " 2.2.2: v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );
    } // second(v)

    public Value third( Value v ) {
        System.out.println( " 3.1.1:  entered: v.i = " + v.i ); // 25
        System.out.println( " 3.1.2:  v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );

        v = new Value();
        v.i = 9;
        System.out.println( " 3.2.1:  created: v.i = " + v.i ); // 9
        System.out.println( " 3.2.2:  v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );

        return v;
    } // third(v)

    public Value fourth( final Value v ) {
        System.out.println( " 4.1.1:entered: v.i = " + v.i ); // 9
        System.out.println( " 4.1.2:v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );

        /**********************************
        // The final local v can't be assigned. It must be blank and not using a compound assignment.
        // meaning, you are not allowed to change its memory location,
        // but can alter its content, if permitted
        // v = new Value();
        //**********************************/

        v.i = 45;
        System.out.println( " 4.2.1:changed: v.i = " + v.i ); // 45
        System.out.println( " 4.2.2:v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );

        return v;
    } // fourth(v)

    public void first() {
        System.out.println( "1.1.1: entered: ..." );

        Value v = new Value();
        System.out.println( "1.2.1: created; v.i = " + v.i ); // 15
        v.i = 25;
        System.out.println( "1.2.2: changed: v.i = " + v.i ); // 25
        System.out.println();

        System.out.println( "1.3.1: before calling second(v) ..." );
        System.out.println( " v.i = " + v.i + ", v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );
        second( v );
        System.out.println( "1.3.2: returning from second(v) ..." );
        System.out.println( " v.i = " + v.i + ", v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );
        System.out.println();

        System.out.println( "1.4.1:  before calling third(v) ..." );
        System.out.println( " v.i = " + v.i + ", v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );
        v = third( v );
        System.out.println( "1.4.2:  returning from third(v) ..." );
        System.out.println( " v.i = " + v.i + ", v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );
        System.out.println();

        System.out.println( "1.5.1: before calling fourth(v) ..." );
        System.out.println( " v.i = " + v.i + ", v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );
        v = fourth( v );
        System.out.println( "1.5.2: returning from fourth(v) ..." );
        System.out.println( " v.i = " + v.i + ", v.hashCode() = " + v.hashCode() + "; v = " + v.toString() );
    } // first()

    public static void main( String ... a ) {
        Test _this = new Test();
        _this.first();
    } // psvm(...)
} // class Test

当您运行上面的示例时，您可能会看到如下输出:

1.1.1: entered: ...
1.2.1: created; v.i = 15
1.2.2: changed: v.i = 25

1.3.1: before calling second(v) ...
 v.i = 25, v.hashCode() = 1671711; v = Test$Value@19821f
 2.1.1: entered: v.i = 25
 2.1.2: v.hashCode() = 1671711; v = Test$Value@19821f
 2.2.1:   new V: v.i = 9
 2.2.2: v.hashCode() = 11394033; v = Test$Value@addbf1
1.3.2: returning from second(v) ...
 v.i = 25, v.hashCode() = 1671711; v = Test$Value@19821f

1.4.1:  before calling third(v) ...
 v.i = 25, v.hashCode() = 1671711; v = Test$Value@19821f
 3.1.1:  entered: v.i = 25
 3.1.2:  v.hashCode() = 1671711; v = Test$Value@19821f
 3.2.1:  created: v.i = 9
 3.2.2:  v.hashCode() = 4384790; v = Test$Value@42e816
1.4.2:  returning from third(v) ...
 v.i = 9, v.hashCode() = 4384790; v = Test$Value@42e816

1.5.1: before calling fourth(v) ...
 v.i = 9, v.hashCode() = 4384790; v = Test$Value@42e816
 4.1.1:entered: v.i = 9
 4.1.2:v.hashCode() = 4384790; v = Test$Value@42e816
 4.2.1:changed: v.i = 45
 4.2.2:v.hashCode() = 4384790; v = Test$Value@42e816
1.5.2: returning from fourth(v) ...
 v.i = 45, v.hashCode() = 4384790; v = Test$Value@42e816

<小时/>

如果您确实想在被调用方法中保存对对象 instanceVariableV 所做的更改，例如 fifth()，另一种可能性是声明 v 作为实例变量。

下面的例子将解释这些差异。

public class Test {
    Value instanceVariableV = null; // v

    // rest of other variables and methods here
        // ...

    public void fifth() {
        System.out.println( " 5.1.1:entered: instanceVariableV = " + instanceVariableV ); // null
        // null, hence no hashCode(), and no toString() will work

        // let us create an instance of Value
        instanceVariableV = new Value();
        System.out.println( " 5.2.1:created: instanceVariableV = " + instanceVariableV ); // Test$Value@9304b1
        System.out.println( " 5.2.2: instanceVariableV.i = " + instanceVariableV.i ); // 15
        System.out.println( " 5.2.3: hashCode = " + instanceVariableV.hashCode() ); // 9634993

        instanceVariableV.i = 20;
        System.out.println( " 5.3.1:changed: instanceVariableV.i = " + instanceVariableV.i ); // 20
        System.out.println( " 5.3.2: hashCode = " + instanceVariableV.hashCode() ); // 9634993 // not changed 
    } // fifth()

    public void first() {
        // continuation of code

        System.out.println( "1.6.1: before calling fifth() ..." );
        System.out.println( " instanceVariableV = " + instanceVariableV  );
        fifth();
        System.out.println( "1.6.2: returning from fifth() ..." );
        System.out.println( " instanceVariableV = " + instanceVariableV  );
        if ( instanceVariableV != null ) {
            // must be different from the one when created new
            System.out.println( " .i = " + instanceVariableV.i );
            // this won't differ
            System.out.println( " .hashCode() = " + instanceVariableV.hashCode() );
        }
    } // first()

    public static void main( String ... a ) {
        // ...
        System.out.println( "\r\nmain(...): vInstanceVariable = " + _this.instanceVariableV );
        if ( _this.instanceVariableV != null ) {
            // must be different from the one when created new
            System.out.println( " .i = " + _this.instanceVariableV.i );
            // this won't differ
            System.out.println( " .hashCode() = " + _this.instanceVariableV.hashCode() );
        }
    } // psvm(...)

当您运行上述扩展示例时，您可能会看到如下输出:

1.6.1: before calling fifth() ...
 instanceVariableV = null
 5.1.1:entered: instanceVariableV = null
 5.2.1:created: instanceVariableV = Test$Value@9304b1
 5.2.2: instanceVariableV.i = 15
 5.2.3: hashCode = 9634993
 5.3.1:changed: instanceVariableV.i = 20
 5.3.2: hashCode = 9634993
1.6.2: returning from fifth() ...
 instanceVariableV = Test$Value@9304b1
 .i = 20, .hashCode() = 9634993

main(...): vInstanceVariable = Test$Value@9304b1
 .i = 20
 .hashCode() = 9634993

希望这对您有帮助。

其他引用:

1. Does Java pass by reference or pass by value?
2. Is java pass by reference? (A posting on SO)