私有变量在类之外被修改 - Java

public class MyClass {
    private int myVar;

    public int getMyVar() {
        return myVar;
    }

    public void setMyVar(int myVar) {
        this.myVar = myVar;
    }
}

public class MyClass {
    private int myVar;

    public int getMyVar() {
        return myVar;
    }

    public void setMyVar(int myVar) {
        this.myVar = myVar;
    }
}

public interface MyInterface {
    void setMyVar(int myVar);
    int getMyVar();
}

public class MyClass implements MyInterface {
    private int myVar;

    @Override
    public void setMyVar(int myVar) {
        this.myVar = myVar;
    }

    @Override
    public int getMyVar() {
        return myVar;
    }
}

import java.lang.reflect.Field;

public class MyClass {
    private int myVar;

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
        MyClass obj = new MyClass();
        Field field = MyClass.class.getDeclaredField("myVar");
        field.setAccessible(true);
        field.setInt(obj, 10);
        System.out.println(field.getInt(obj));
    }
}