Паттерн Компоновщик (Composite). Пример кода

/**
 * Component
 */
public abstract class TextPart {
    
    protected List value = new ArrayList();
    
    public String getValue() {
        StringBuilder sb = new StringBuilder();
        for (TextPart textPart : value) {
            sb.append(textPart.getValue());
        }
        return sb.toString();
    }
    
    public void append(TextPart... textPart) {
        for (TextPart part : textPart) {
            value.add(part);
        }
    }
}

/**
 * Composite
 */
public class Sentence extends TextPart {
    public Sentence(TextPart... value) {
        for (TextPart textPart : value) {
            this.value.add(textPart);
        }
    }
}

/**
 * Composite
 */
public class Paragraph extends Sentence {

    public Paragraph(TextPart... value) {
        super(value);
    }

    @Override
    public String getValue() {
        return "    " + super.getValue() + "\n\n";
    }

}


/**
 * Leaf
 */
public class Symbol extends TextPart {
    private char value;
    
    public Symbol(char value) {
        this.value = value;
    }

    @Override
    public String getValue() {
        return String.valueOf(value);
    }

    @Override
    public void append(TextPart... textPart) {
        throw new UnsupportedOperationException();
    }
}


public class Client {
    public static void main(String[] args) {
        Sentence sentence1  = new Sentence(
            new Symbol('H'),
            new Symbol('e'),
            new Symbol('l'),
            new Symbol('l'),
            new Symbol('o')
        );
        System.out.println(sentence1.getValue());
        
        Sentence sentence2  = new Sentence(
            new Symbol('w'),
            new Symbol('o'),
            new Symbol('r'),
            new Symbol('l'),
            new Symbol('d'),
            new Symbol('!')
        );
        
        Sentence sentence3 = new Sentence(sentence1, new Symbol(' '), sentence2);
        System.out.println(sentence3.getValue());
        
        Paragraph paragraph = new Paragraph(sentence3, new Symbol(' '), sentence3);
        System.out.println(paragraph.getValue());
    }
}

/**
 * Output:
 * Hello
 * Hello world!
 *     Hello world! Hello world!
 */