this Keyword

public class Dog {
    private String name;
    
    public Dog(String name) {
        this.name = name;   // this.name = the instance variable
                            // name = the parameter
    }
    
    public void introduce() {
        System.out.println("I am " + this.name);
        // same as: System.out.println("I am " + name);
    }
}

Dog d = new Dog("Max");
d.introduce();   // "I am Max" — this refers to d

public class Point {
    private int x;
    private int y;
    
    // Without this — WRONG!
    public Point(int x, int y) {
        x = x;   // assigns parameter to itself! Instance variable unchanged!
        y = y;   // same problem!
    }
    
    // With this — CORRECT!
    public Point(int x, int y) {
        this.x = x;   // this.x = instance variable, x = parameter
        this.y = y;   // this.y = instance variable, y = parameter
    }
}

public Point(int x, int y) {
    x = x;   // parameter x = parameter x (useless)
}

Point p = new Point(3, 5);
// p.x is still 0! The instance variable was never set!

public class Circle {
    private double radius;
    
    // No conflict — this is optional
    public double getArea() {
        return Math.PI * radius * radius;
        // same as: return Math.PI * this.radius * this.radius;
    }
}

public void setRadius(double radius) {
    this.radius = radius;   // this is REQUIRED here
}

public class Student {
    private String name;
    
    public Student(String name) {
        this.name = name;
    }
    
    public void enrollIn(Course course) {
        course.addStudent(this);   // pass THIS student to the course
    }
}

public class Builder {
    private String name;
    private int age;
    
    public Builder setName(String name) {
        this.name = name;
        return this;   // returns the current object
    }
    
    public Builder setAge(int age) {
        this.age = age;
        return this;
    }
}

// Method chaining
Builder b = new Builder().setName("Alice").setAge(20);

public class Circle {
    private double radius;
    
    public Circle(double radius) {
        this.radius = radius;
    }
    
    public boolean isBiggerThan(Circle other) {
        return this.radius > other.radius;
        // this = the circle calling the method
        // other = the circle passed as argument
    }
}

Circle c1 = new Circle(5);
Circle c2 = new Circle(3);
c1.isBiggerThan(c2);  // this = c1, other = c2 → true
c2.isBiggerThan(c1);  // this = c2, other = c1 → false

public class Counter {
    private int value;
    
    public Counter(int value) {
        this.value = value;
    }
    
    public Counter add(Counter other) {
        this.value += other.value;
        return this;
    }
    
    public int getValue() { return value; }
}

Counter a = new Counter(10);
Counter b = new Counter(5);
Counter c = new Counter(3);

a.add(b).add(c);   // chain!