Let's say we have the following class:

public class Phone{
    private String os;
    private String processor;
    private int battery;
    private int camera;

    public Phone(String os, String processor, int battery, int camera){
        super();
        this.os = os;
        this.processor = processor;
        this.battery = battery;
        this.camera = camera;
    }
}

Now when we have to create a Object of this class, we have to pass all the 4 arguments in the correct order. This is a sort of a headache. To avoid this passing of values, we can use a PhoneBuilder class.

public class PhoneBuilder{
    private String os;
    private String processor;
    private int battery;
    private int camera;

    public PhoneBuilder setOs(String os){
        this.os = os;
        return this;
    }
    public PhoneBuilder setProcessor(String processor){
        this.processor = processor;
        return this;
    }
    public PhoneBuilder setBattery(int battery){
        this.battery = battery;
        return this;
    }
    public PhoneBuilder setCamera(int camera){
        this.camera = camera;
        return this;
    }
    public Phone getPhone(){
        return new Phone(os, processor, battery, camera);
    }
}

We have setters, all which return PhoneBuilder. We have a getPhone method as well.

Now how do we use this?

public class Shop{
    Phone p = new PhoneBuilder().setOs("Android").setBattery(1000).getPhone();
}

Now we have instantiated the class, without giving all args and in correct order. We can choose to give only a few Args and in any order that we like!