为什么Java中的某些方法会修改原始值而有些则不会？

public static void main(String[] args) {
    int a = 10;
    System.out.println("Before: " + a);
    modify(a);
    System.out.println("After: " + a);
}

public static void modify(int a) {
    a = 20;
}

Before: 10
After: 10

public static void main(String[] args) {
    MyClass a = new MyClass(10);
    System.out.println("Before: " + a.getValue());
    modify(a);
    System.out.println("After: " + a.getValue());
}

public static void modify(MyClass a) {
    a.setValue(20);
}

Before: 10
After: 20