Basic interfaces in java.lang

Basic interfaces in java.lang

Common Interfaces in java.lang

InterfacePurposeKey Methods
AppendableAllows an object to receive appended character sequences. Implemented by classes like StringBuilder, StringBuffer, Writer.append(CharSequence csq), append(char c)
CharSequenceRepresents a readable sequence of char values. Implemented by String, StringBuilder, StringBuffer, CharBuffer.length(), charAt(int index), subSequence(int start, int end)
CloneableMarks classes whose objects can be cloned using Object.clone(). (Marker interface — no methods).(No methods)
Comparable<T>Defines a natural ordering for objects of a class. Used in sorting. Implemented by String, Integer, Double, etc.compareTo(T o)
Iterable<T>Allows an object to be the target of an enhanced for loop (for-each). Implemented by Collection, List, Set, etc.iterator()
ReadableRepresents a source of characters that can be read into a CharBuffer.read(CharBuffer cb)
RunnableRepresents a task that can be executed by a thread. Implemented by Thread and custom task classes.run()

Quick Examples

Comparable

class Student implements Comparable<Student> {
    int marks;
    Student(int marks) { this.marks = marks; }

    @Override
    public int compareTo(Student other) {
        return Integer.compare(this.marks, other.marks);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student(80);
        Student s2 = new Student(90);
        System.out.println(s1.compareTo(s2)); // negative (less than)
    }
}

Runnable

class MyTask implements Runnable {
    @Override
    public void run() {
        System.out.println("Task is running");
    }
}

public class Main {
    public static void main(String[] args) {
        Thread t = new Thread(new MyTask());
        t.start();
    }
}

Marker Interfaces

Some interfaces like Cloneable have no methods — they are “marker interfaces” used by the JVM or APIs to check if a class has special capabilities.

✅ Summary

  • java.lang interfaces are fundamental building blocks.
  • Some define contracts (Comparable, Runnable), others are markers (Cloneable).
  • Many are implemented by core Java classes (String, Thread, StringBuilder, etc.).

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