3.9 this Keyword

What you need to know

• Inside an instance method or constructor, this is a reference to the current object — the one the method was called on.
• this.name = name; assigns the parameter to the instance variable when they share a name. Without this, the line would assign the parameter to itself.
• this can be passed as an argument: other.compareTo(this), or used to return the current object.
• this cannot be used in a static method — there's no current object.
• Calling a method with no object prefix inside a class is the same as this.method().
• Using this is optional when there's no shadowing; it's only required to disambiguate.

Worked example

public class Book
{
    private String title;
    private int pages;

    public Book(String title, int pages)
    {
        this.title = title;      // instance ← parameter
        this.pages = pages;
    }

    public boolean isLongerThan(Book other)
    {
        return this.pages > other.pages;
    }
}

Without this. in the constructor, title = title; assigns the parameter to itself and the instance variable stays null.

Going deeper

The nuance, edge cases, and connections that turn a 3 into a 5.

• this is an implicit reference variable available in every instance method and constructor. It points to the object the method was called on. a.getName() → inside getName, this is a.
• The main use: resolving shadowing in constructors and setters. this.name = name; assigns the parameter to the field. Without this, both sides refer to the parameter.
• Passing this: a method can hand the current object to another method: other.merge(this), or list.add(this). Useful when an object needs to register itself somewhere.
• Returning this enables chaining, though the exam doesn't test that pattern.
• Comparing with this: in an equals or comparison method, this.value == other.value compares the current object's field with the parameter object's field. Both are the same class, so the method can access other's private fields directly.
• Static methods have no this — there's no object. Using it there is a compile error.
• Inside an instance method, a bare instance variable name is shorthand for this.name. The two are identical when there's no shadowing.

Mistakes that cost points

• Omitting this when the parameter shadows the field. Silent bug — the field stays default (null or 0).
• Using this in a static context. Compile error.
• Thinking this is a special variable you declare. It's provided automatically.

Practice questions

Written in the style of the real exam. Try each one before revealing the answer.

public Cat(String name)
{
    name = name;
}