Common operations- Stream Java8

Streams API in Java8 provides good number of Stream operations to operate on input provided. These operationsreduce lot of code to be written and increases readability.Some of the most important functions are described below with examples

• Collect(toList()) :- is an eager operation which generates list from a given stream

        Example :- Below example describes the use of collect  operation which gets the  list of skills  from employees

/**
package com.test;

import java.util.ArrayList;

/**
 * @author sudheer
 *
 */
public class Tester {

	public Employee createEmployee(int employeeId, float experience, String firstName, String lastName, String skill) {
		Employee emp = new Employee();
		emp.setEmployeeId(employeeId);
		emp.setExperience(experience);
		emp.setFirstName(firstName);
		emp.setLastName(lastName);
		emp.setSkill(skill);
		return emp;

	}

	public static void main(String[] args) {
		Tester test = new Tester();
		List<Employee> employees = new ArrayList<Employee>();
		employees.add(test.createEmployee(1, 5, "Sudheer", "Reddy", "Java"));
		employees.add(test.createEmployee(2, 4, "Keerthi", "Reddy", "Java"));
		employees.add(test.createEmployee(3, 7, "Loknath", "Reddy", ".Net"));
		employees.add(test.createEmployee(4, 5, "Satish", "Reddy", "C++"));

		// Applying eager operation count
		employees.stream().filter(employee -> {
			System.out.println("Employee skill->" + employee.getSkill());
			return "Java".equals(employee.getSkill());
		}).count();

		// Lazy operations .output is not printed on console ,because traversal
		// of the filtered stream is not done until eager operation is called
		employees.stream().filter(employee -> {
			System.out.println("Employee skill->" + employee.getSkill());
			return "Java".equals(employee.getSkill());
		});

	}

}

• map :- 

•  used to replace values in a stream with new values like converting stream of lowercase letter array to uppercase letter array
• Returns a stream consisting of the results of applying the given function to the elements of this stream. This is an intermediate operation.
• its a lazy operation  which will  return output as Stream object.

         Example :- In the above example map is used to get/convert stream of employees into an intermediate stream of Skills

List<String> list = employees.stream().map(Employee::getSkill).collect(Collectors.toList());

• Filter :- 

• This operation is used to filter out the result by keeping some elements in  Stream and throwing out the other elements.
• is used  to get required values from a Stream 
• This is a lazy operation , traversal of intermediate new Stream will not be started until eager operation is called  .

        Example :- Below example describes the use of collect  operation which gets the  list of skills  from employees

package com.test;

import java.util.ArrayList;

/**
 * @author sudheer
 *
 */
public class Tester {

	public Employee createEmployee(int employeeId, float experience, String firstName, String lastName, String skill) {
		Employee emp = new Employee();
		emp.setEmployeeId(employeeId);
		emp.setExperience(experience);
		emp.setFirstName(firstName);
		emp.setLastName(lastName);
		emp.setSkill(skill);
		return emp;

	}

	public static void main(String[] args) {
		Tester test = new Tester();
		List<Employee> employees = new ArrayList<Employee>();
		employees.add(test.createEmployee(1, 5, "Sudheer", "Reddy", "Java"));
		employees.add(test.createEmployee(2, 4, "Keerthi", "Reddy", "Java"));
		employees.add(test.createEmployee(3, 7, "Loknath", "Reddy", ".Net"));
		employees.add(test.createEmployee(4, 5, "Satish", "Reddy", "C++"));

		// Applying eager operation count
		employees.stream().filter(employee -> {
			System.out.println("Employee skill->" + employee.getSkill());
			return "Java".equals(employee.getSkill());
		}).count();

		// Lazy operations .output is not printed on console ,because traversal
		// of the filtered stream is not done until eager operation is called
		employees.stream().filter(employee -> {
			System.out.println("Employee skill->" + employee.getSkill());
			return "Java".equals(employee.getSkill());
		});

	}

}

Note :- In above example  @ 1 the out put is printed as  below this is due to call of eager function count(...)

Employee skill->Java

Employee skill->Java

Employee skill->.Net

Employee skill->C++

@ 2 there is no eager function called so no output is printed when executed.


In next post we will discuss about flatMap,reduce,sort etc. Stream operations
Happy learning