更改通过参数传递的值

Question

我正在测试是否可以通过传递参数来更改静态变量 x 的值，但我发现你不能那样做。

public class Test {
static int x;

static void changeX(int x_) {
    x_ = 50;

}

public static void main(String args[]) {

    changeX(x);
    System.out.println(x);//it prints out zero because the static variable did not get changed


}

}

如果我们这样做，我们可以改变它：

public class Test {
static int x;

static void changeX(int x_) {
    x = 50;

}

public static void main(String args[]) {

    changeX(x);
    System.out.println(x);

}

}

这意味着您必须直接引用静态变量才能更改它。好的。现在。我的问题是，有没有办法通过仅通过参数传递它来更改 class 变量，而无需在实现中引用它？基本上，有没有办法以某种方式使用第一种方式？谢谢

Answer 1

Java 始终将基元作为值传递。另一方面，对象总是作为引用传递。在第二个示例中，您访问了静态属性 x 而不是参数 x_。此外，static 不保护属性不被重写。它将属性绑定到 class（没有 static 属性绑定到对象）。也许你的意思是 final?

编辑：更正了一个打字错误。

Answer 2

您可以实现您所要求的，但您需要注意细微的限制。您可以修改传递给方法的对象引用，但不能重新分配方法中的引用并更改原始对象。

Java 总是按值传递。时期。对象引用按值传递，但正是通过这些引用（指向相同的内存位置），您可以通过参数修改方法内部的对象。

您不能以这种方式修改基元（int、float、boolean等），它们总是按值传递.您也不能修改不可变对象（例如 String），因为它们不能使用 public 接口进行更改。

考虑这个简单的示例，它测试创建一个具有一些可修改字段、按值和按引用（它通过对象引用修改，它本身按值传递）的对象：

public class ParameterPassingTest {
    static SomeObject someStaticObject;

    public static void main(String[] args) {
        // initialize a static object reference
        someStaticObject = new SomeObject("I am a static Object", 10);

        System.out.println("My static object before: " + someStaticObject);

        // try modifying the reference by value
        modifySomeObjectByValue(someStaticObject);
        // try printing the value, it will be the same
        System.out.println("My static object after mod-by-value: " + someStaticObject);

        // now, try modifying by object reference
        modifySomeObjectByReference(someStaticObject);

        // print again. new values should be observed
        System.out.println("My static object after mod-by-reference: " + someStaticObject);
    }

    // this method tries to modify the original object by assigning directly to the method parameter. It won't work.
    public static void modifySomeObjectByValue(SomeObject someObject) {
        SomeObject newObject = new SomeObject("I am another object, from a local method", 20);
        someObject = newObject; // try to modify the original object by assigning to the parameter directly
    }

    // this method tries to modify the original object by using the object's public interface. It works.
    public static void modifySomeObjectByReference(SomeObject someObject) {
        // try to modify the original object by using the reference passed in
        someObject.setaString("I have been modified by a method");
        someObject.setAnInt(50);
    }
}

// simple, generic class object with some fields.
class SomeObject {
    String aString;
    int anInt;

    public SomeObject(String aString, int anInt) {
        this.aString = aString;
        this.anInt = anInt;
    }

    public String getaString() {
        return aString;
    }

    public void setaString(String aString) {
        this.aString = aString;
    }

    public int getAnInt() {
        return anInt;
    }

    public void setAnInt(int anInt) {
        this.anInt = anInt;
    }

    @Override
    public String toString() {
        return "aString = " + getaString() + " | anInt = " + getAnInt();
    }
}

这会产生输出：

My static object before: aString = I am a static Object | anInt = 10
My static object after mod-by-value: aString = I am a static Object | anInt = 10
My static object after mod-by-reference: aString = I have been modified by a method | anInt = 50

更改通过参数传递的值

Changing the value being passed through the parameter

java

parameters

reference

parameter-passing