In java 8, predicate is an functional interface java.util.function.Predicate<T> which defines an abstract method named test, that accepts an object of generic type T and returns a boolean. Predicate can be used as assignment target for a lambda expression or method reference.

Example:

package com.manishsanger.Predicate;

public class Vehicle {

private String type;

private Integer displacement;
public Vehicle(String type, Integer displacement) {

this.type = type;

this.displacement = displacement;

}
@Override

public String toString() {

return "Vehicle - Type:" + this.getType() + ", Displacement:" + this.getDisplacement() +"cc";

}
public String getType() {

return type;

}
public void setType(String type) {

this.type = type;

}
public Integer getDisplacement() {

return displacement;

}
public void setDisplacement(Integer displacement) {

this.displacement = displacement;

}

}

If we need to get the vehicle whose displacement is more that 1500 cc, we can write a predicate for it as below:

public class VehiclePredicate {

public static Predicate<Vehicle> isDisplacementAbove1500() {

return vehicle -> vehicle.getDisplacement() >= 1500;

}

}

We can user this predicate in filter(), to filter list of vehicle by condition in the predicate.

package com.manishsanger.Predicate;

import java.util.Arrays;

import java.util.List;

import java.util.stream.Collectors;
public class Example {
public static void main(String[] args) {

List<Vehicle> vehicles = Arrays.asList(

new Vehicle("SUV", 2200),

new Vehicle("SUV", 1800),

new Vehicle("SEDAN", 1600),

new Vehicle("HATCHBACK", 1400),

new Vehicle("SUV", 2800)

);
//Using default filter() method, using stream and lambda expression.
//return all Vehicles those have displacement more than 1500

System.out.println("All vehicles those have displacement more than 1500, Using default filter() method.");

vehicles.stream().filter(vehicle -> {

return vehicle.getDisplacement() >= 1500;

}).collect(Collectors.toList()).forEach(System.out::println);
//return all Vehicles those have displacement more than 1500

System.out.println("All SUV those have displacement more than 1500, Using default filter() method.");

vehicles.stream().filter(vehicle -> {

return vehicle.getType().equals("SUV") && vehicle.getDisplacement() >= 1500;

}).collect(Collectors.toList()).forEach(System.out::println);
}

}

Following will be the output:

All vehicles those have displacement more than 1500, Using default filter() method.

Vehicle - Type:SUV, Displacement:2200cc

Vehicle - Type:SUV, Displacement:1800cc

Vehicle - Type:SEDAN, Displacement:1600cc

Vehicle - Type:SUV, Displacement:2800cc

All SUV those have displacement more than 1500, Using default filter() method.

Vehicle - Type:SUV, Displacement:2200cc

Vehicle - Type:SUV, Displacement:1800cc

Vehicle - Type:SUV, Displacement:2800cc

We can also define our own custom filter method(), Now add a filterVehicles() method in VehiclePredicate class.

public static List<Vehicle> filterVehicles(List vehicles, Predicate<Vehicle> predicate) {

return vehicles.stream().filter(predicate).collect(Collectors.toList());

}

Add following code to Main class, to filter data using custom Predicate filter:

//Using custom filter method and java 8 lambda

System.out.println("\nAll vehicles those have displacement more than 1500, Using default custom filter method.");

VehiclePredicate.filterVehicles(vehicles, vehicle -> {

return vehicle.getDisplacement() >= 1500;

}).forEach(System.out::println);

Here is the output:

All vehicles those have displacement more than 1500, Using default custom filter method.

Vehicle - Type:SUV, Displacement:2200cc

Vehicle - Type:SUV, Displacement:1800cc

Vehicle - Type:SEDAN, Displacement:1600cc

Vehicle - Type:SUV, Displacement:2800cc

Here is how you can filter the list without lambda expression:

//Using custom filter method and without lambda

System.out.println("\nAll vehicles those have displacement more than 1500, Using default custom filter method without lambda.");

VehiclePredicate.filterVehicles(vehicles, VehiclePredicate.isDisplacementAbove1500()).forEach(System.out::println);

Here is the output, for the above block of code:

All vehicles those have displacement more than 1500, Using default custom filter method.

Vehicle - Type:SUV, Displacement:2200cc

Vehicle - Type:SUV, Displacement:1800cc

Vehicle - Type:SEDAN, Displacement:1600cc

Vehicle - Type:SUV, Displacement:2800cc

Java 8 Predicate Methods
Let’s now go through the methods available for Predicate:

1. Default Predicate and (Predicate other)

It returns a composed predicate that represents a logical AND of two predicates.

To understand this let’s add another predicate in the VehiclePredicates class:

public static Predicate<Vehicle> isTypeSUV() {

return vehicle -> vehicle.getType().equals("SUV");

}

Now we can apply and() and or() function if we want to predicate for Vehicle of type SUV and displacement more than 1500 cc.

Here is the code for Main method:

//Multiple filter with AND logical operator

System.out.println("\nAll SUV those have displacement more than 1500, Using custom Predicate filter without lambda.");

VehiclePredicate.filterVehicles(vehicles, VehiclePredicate.isDisplacementAbove1500().and(VehiclePredicate.isTypeSUV())).forEach(System.out::println);

//Multiple filter with OR logical operator

System.out.println("\nAll Vehicle those have displacement more than 1500 or SUV, Using custom Predicate filter without lambda.");

VehiclePredicate.filterVehicles(vehicles, VehiclePredicate.isDisplacementAbove1500().or(VehiclePredicate.isTypeSUV())).forEach(System.out::println);

Here is the output:

All SUV those have displacement more than 1500, Using custom Predicate filter without lambda.

Vehicle - Type:SUV, Displacement:2200cc

Vehicle - Type:SUV, Displacement:1800cc

Vehicle - Type:SUV, Displacement:2800cc

All Vehicle those have displacement more than 1500 or SUV, Using custom Predicate filter without lambda.

Vehicle - Type:SUV, Displacement:2200cc

Vehicle - Type:SUV, Displacement:1800cc

Vehicle - Type:SEDAN, Displacement:1600cc

Vehicle - Type:SUV, Displacement:2800cc

2. Default Predicate negate()
It return an predicate that represents logical NOT operator.

For example all Vehicles other than SUV:

//default negate(), Logical NOT operator. All vehicles other than SUV

System.out.println("\nAll vehicles other than SUV, using negate()");

VehiclePredicate.filterVehicles(vehicles, VehiclePredicate.isTypeSUV().negate()).forEach(System.out::println);

Here is the output:

All vehicles other than SUV, using negate()

Vehicle - Type:SEDAN, Displacement:1600cc

Vehicle - Type:HATCHBACK, Displacement:1400cc

3. Boolean test (T t)
Predicate test evaluates this predicate on the given argument.
For example we can pass an Vehicle object to check if this predicate return true or false:

Add the following code in main method:

//boolean test(T t) predicate

System.out.println("\nboolean test predicate");

Vehicle testVehicle = new Vehicle("SUV", 1000);

System.out.println(VehiclePredicate.isTypeSUV().test(testVehicle));

System.out.println(VehiclePredicate.isDisplacementAbove1500().test(testVehicle));

Here is the output:

boolean test(T t) predicate

true

false

4. static Predicate isEqual(Object targetRef)
Returns a predicate that tests if two arguments are equal according to Objects.equals() method.

Let’s say we overridden equals() method for Vehicle class

@Override

public boolean equals(Object obj) {

Vehicle vehicle = (Vehicle) obj;

if (this.getType().equals(vehicle.getType()) && this.getDisplacement().equals(vehicle.getDisplacement())) {

return true;

}

return false;

}

Now let’s say we have a Vehicle which has standard type & displacement. Then we can get a Predicate, that will test if the given Vehicle is standard or not.

//static Predicate isEqual(Object targetRef)

System.out.println("\nstatic Predicate isEqual(Object targetRef)");

Predicate standardVehiclePredicate = Predicate.isEqual(new Vehicle("SUV", 1800));

System.out.println(standardVehiclePredicate.test(new Vehicle("SUV", 1800)));

System.out.println(standardVehiclePredicate.test(new Vehicle("SUV", 1500)));

Here is the output:

static Predicate isEqual(Object targetRef)

true

false

Click here to see the source code

The post Predicate – Functional Interface (Java 8) appeared first on Manish Sanger.

Java introduced Functional interface & Lambda Expressions in Java 8, to leverage functional programming to reduce verbosity.

What is functional Interface?

A Functional Interface is an interface which specifies exactly one abstract method. Its implementation may be treated as Lambda Expression.

Note: An interface is still a functional interface if it has many default methods, as long as it specifies only one abstract method.

Since default methods have an implementation, they are not abstract. If an interface declares an abstract method overriding one of the public methods of java.lang.Object, that also does not count toward the interface’s abstract method count since any implementation of the interface will have an implementation from java.lang.Object or elsewhere

While the main use of Functional interfaces is for lambda expressions, method references, and constructor references, they can still be used, like any interface, with anonymous classes, implemented by classes, or created by factory methods.

Java 8 provides some inbuilt functional interfaces like Comparable, Runnable, Callable etc.

If you want to define any functional interface of your own, you can use annotation @FunctionalInterface for compiler level errors. It will allow to specify only one abstract method.

Example:

@FunctionalInterface

public interface SomeInterface {

public String someMethod(String param);

}

A functional interface is valid even if the @FunctionalInterface annotation would be omitted. It is only for informing the compiler to enforce single abstract method inside interface.

Public interface SomeInterface {

String someMethod(String param);

}

//With default method

@FunctionalInterface

Public interface SomeInterface {

public String someMethod(String param);

default void someDefaultMethod(){

//method Implementation

}
}
// When overriding methods from the java.lang.Object
@FunctionalInterface

Public interface SomeInterface {

public String someMethod(String param);

@Override

public String toString();  //Overridden from Object class
@Override

public boolean equals(Object obj); //Overridden from Object class
}

The signature of abstract method in functional interface is called Functional descriptor because it defines the signature of the lambda expression.

Java 8 has introduced several functional interfaces in java.util.function package which can be used to describe the signature of various lambda expressions.

Here is the complete list of functional interfaces available in java.util.functional:

1. Predicate
2. Consumer
3. Function<T, R>
4. Supplier
5. UnaryOperator
6. BinaryOperator
7. BiPredicate<L, R>
8. BiConsumer<T, U>
9. BiFunction<T, U, R>

The post Functional Interface – Java 8 appeared first on Manish Sanger.

New features introduced in java 8

Here are the list of high level features introduced in Java 8 along with lots of enhancement at complier and JVM level:

1. Lambda Expressions
Lambda expression is a concise representation of an anonymous function which can be passed as an argument to a method or stored in a variable. Click Here more details

2. Functional Interface
A Functional Interface is an interface which specifies exactly one abstract method. Click Here more details
3. Method References
4. Default methods in interface
5. Optional
6. Stream API
7. Date API
8. Nashorn, JavaScript Engine

The post What new features were added in Java8? appeared first on Manish Sanger.