The Builder interface is designed to designate a class as a builder object in the Builder design pattern. Builders are capable of creating and configuring objects or results that normally take multiple steps to construct or are very complex to derive.

The builder interface defines a single method, build(), that classes must implement. The result of this method should be the final configured object or result after all building operations are performed.

It is a recommended practice that the methods supplied to configure the object or result being built return a reference to this so that method calls can be chained together.

Example Builder:

class FontBuilder implements Builder<Font> {
    private Font font;

    public FontBuilder(String fontName) {
        this.font = new Font(fontName, Font.PLAIN, 12);
    }

    public FontBuilder bold() {
        this.font = this.font.deriveFont(Font.BOLD);
        return this; // Reference returned so calls can be chained
    }

    public FontBuilder size(float pointSize) {
        this.font = this.font.deriveFont(pointSize);
        return this; // Reference returned so calls can be chained
    }

    // Other Font construction methods

    public Font build() {
        return this.font;
    }
}

Example Builder Usage:

Font bold14ptSansSerifFont = new FontBuilder(Font.SANS_SERIF).bold()
                                                             .size(14.0f)
                                                             .build();