3.9 this Keyword

What is the this keyword?

this is a reference to the current object — the instance whose method or constructor is running. It's used inside a class to refer to instance variables or invoke other constructors, especially when a parameter name shadows an instance variable name.

Resolving name conflicts

When a constructor or method parameter has the same name as an instance variable, Java assumes the local parameter unless you use this.variableName. Writing `this.age = age;` tells Java to assign the parameter value to the object's instance variable, not itself.

Calling other constructors

this(...) can be used as the first statement in a constructor to call another constructor in the same class, avoiding duplicate code. This is called constructor chaining — one constructor sets defaults, and another calls it with `this(...)` to reuse that logic.

Key terms

this keyword

A reference to the current object executing a method or constructor.

Why use this.variableName?

To distinguish an instance variable from a parameter or local variable with the same name.

this(...) constructor call

Calls another constructor in the same class; must be the first statement in a constructor.

Constructor chaining

Using this(...) to have one constructor invoke another, reducing duplicate initialization code.

What happens without 'this' when names conflict?

The parameter shadows the instance variable, so the instance variable is never updated.

Try an AP-style question

private int size;
public void setSize(int size) {
    this.size = size;
}

What does this.size refer to?

1. The parameter
2. The instance variable
3. A static variable
4. A new object