Java Functional Interface.

Java functional interface

  • A functional interface is simply an interface that has exactly one abstract method.
  • A functional interface is to be used by Lambda Expression.
  • The following types of members in an interface do NOT count for defining a functional interface:
    • Default methods.
    • Static methods.
    • Public methods inherited from the Object class.
    • Constants.
    @FunctionalInterface // This is not required 
    public interface Percentage { 
      static final double MAX = 100, MIN = 0; // It is allowed to have  constants
      public double getPercentage(double amounts, double value);  // the abstract method
      // It is allowed to override methods in Object class 
      public boolean equals(Object obj);  
      default public double getMaxAdjusted() { // It is allowed to have default methods
        return MAX - 10;
      static public double getMinPercentage() { // It is allowed to have static methods
        return MIN + 10;
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