如何在不同方法中设置的主方法中获取私有变量？

Question

I am trying to change private variables in a method and then access them with a getter method in the main method (as shown below), but when I get the private variable in the main method it is always 0. How can I set a private variable in a method other than the main, but still be able to access that variable in the main method?

public class Example {

    private int testNumber;

    public static void main(String[] args) {
        Example tester = new Example();
        System.out.println(tester.getTestNumber());
        tester.TheTestExample();
        System.out.println(tester.getTestNumber());
    }

    public int getTestNumber(){
        return testNumber;
    }

    public void setTestNumber(int x){
        this.testNumber = x;
    }

    public void TheTestExample(){
        Example MyTester = new Example();
        MyTester.setTestNumber(4);
        System.out.println(MyTester.getTestNumber());
    }
}

Answer 1

There are two separate instances of Example created by your application. One is created by main , and the other by TheTestExample

What you seem to be asking is how main can get the reference to the Example instance in the MyTester variable in your version of the TheTestExample method.

The answer is that it can't. You cannot access a local variable outside of its scope. The method needs to return the reference in that variable, and the caller needs to save it or use it directly. For example.

    tester = tester.theTestExample();
    System.out.println(tester.getTestNumber());

public Example theTestExample(){
    Example myTester = new Example();
    MyTester.setTestNumber(4);
    System.out.println(MyTester.getTestNumber());
    return myTester;
}

By the way:

• MyTester is not a private variable. It is a local variable. Local variables don't / cannot have an access modifier. Calling them "private" is technically incorrect, and confusing. (For others, and probably for you as well.)

• MyTester and TheTestExample are egregious violations of Java style conventions. Method and variable names should always start with a lower case letter.

Answer 2

In your code your are creating 2 objects for Test class one inside main method and another on in TheTestExample() method. And you are setting value 4 in side TheTestExample () for the private variable.

If you want to print the value which was set in TheTestExample() in main () you can return the newly set value from this method. public class Example {

private int testNumber;

public static void main(String[] args) {
    Example tester = new Example();
    System.out.println(tester.getTestNumber());


    **System.out.println(tester.TheTestExample());** // it prints the value return from the method
}

public int getTestNumber(){
    return testNumber;
}

public void setTestNumber(int x){
    this.testNumber = x;
}

public int TheTestExample(){
    Example MyTester = new Example();
    MyTester.setTestNumber(4);
    System.out.println(MyTester.getTestNumber());
    **return MyTester.getTestNumber();**
}

}

Answer 3

public class Example {
private int testNumber;

public static void main(String[] args) {
    Example tester = new Example();
    System.out.println(tester.getTestNumber()); //prints "0"
    tester.TheTestExample(); //print "4"
    System.out.println(tester.getTestNumber()); //print "0" Why? ;) See the method TheTestExample -> print4() for now it is a better name
}

public int getTestNumber(){
    return testNumber;
}

public void setTestNumber(int x){
    this.testNumber = x;
}

public void TheTestExample(){
    Example MyTester = new Example(); //local instance, @Answer we lost  this object. If you want to change testNumber here. If you want to have access to this object please return and use in main method 
    MyTester.setTestNumber(4); //setFour
    System.out.println(MyTester.getTestNumber()); //result in line 8
}

}