This and super keyword usage in android

Question

I am new to android and java and i can not get my head around understanding why in android "this" keyword is sometimes used and not "super"?

Also how can you call parent methods without "this" or "super" keyword.

For example lets say we have

public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

public void sendMessage(View view) {
    Intent intent = new Intent(this, DisplayMessageActivity.class);
    EditText editText = (EditText) findViewById(R.id.edit_message);
    String message = editText.getText().toString();
    intent.putExtra(EXTRA_MESSAGE, message);
    this.startActivity(intent);
}

We can see that in above example MainActivety class does not have setContentView method in fact this method is defined in ActionBarActivity as

public void setContentView(int layoutResID) {
    this.getDelegate().setContentView(layoutResID);
}

So how are we able to call it without "this" or "super"?

Why does it still work if we would put "this" infront although the method is defined in parent class isnt "this" keyword meant for current scope/class.

An expression of a subtype always has access to accessible supertype methods. — Sotirios Delimanolis, Jun 27 '16 at 17:50
You should first take a look at the Java documentation: https://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html, https://docs.oracle.com/javase/tutorial/java/IandI/super.html — Tyler, Jun 27 '16 at 17:50
You also should read up on inheritance my friend https://docs.oracle.com/javase/tutorial/java/IandI/subclasses.html — lucasvw, Jun 27 '16 at 17:50
Possible duplicate of [In Java, what is the difference between this.method() and method()?](http://stackoverflow.com/questions/6547310/in-java-what-is-the-difference-between-this-method-and-method) — anhoppe, Jun 27 '16 at 18:22
There is no difference between 'this.method()' and 'method()'. — Lew Bloch, Jun 27 '16 at 19:33
You're right. If `method` is part of the containing class, then the inner class can call `method()` but not `this.method()`. Good call. I stand corrected. — Lew Bloch, Jul 14 '16 at 22:39

Jean-Baptiste Yunès · Accepted Answer · 2016-06-28T11:47:58.833

How can you call parent methods without "this" or "super" keyword?

You can without super provided that the method you call is not redefined in the "current" class, if it is redefined you may need to disambiguate the call:

class A {
  public void f() {}
  public void h() {} 
  public void i() {} 
}

class B extends A {
  public void g() { f(); } // same as super.f()
  public void h() { h(); /* recursive call */ }
  public void i() { super.i(); /* call to inherited but "masked" */ }
}

how are we able to call it without "this" or "super"?

this is not mandatory, when you write something in an instance method, it is read as this.something. This is why in the preceding example, the call h() is a recursive one.

this is not scoped, it denotes the "full" object itself. super scopes what follows. this is typed with the current class, and super is the object typed as its parent class.

score 0 · Answer 2 · answered Jun 27 '16 at 18:02

In Java, you don't have to explicitly add this every single time you want to access a field or invoke a method in your derived class or the parent class. Extending a class basically means inheriting all the properties of the parent class, while creating a completely new type. The this keyword refers to the instance that's created when instantiating through MyClass instance = new MyClass(), not the MyClass definition. If you want to set a variable on the class scope, you can use a static field.

However, when you have a local variable defined, lets say as an argument to a method (void method(String startActivity){}), then that local definition will override access to the definition in this, therefor requiring the explicit this keyword.

As for the super call, when you override a method in Java you can call the original method that you have overridden by using super.<the method name>(). This is needed because in the class there's no longer a notion of the original method.

This and super keyword usage in android

2 Answers2