toString(); This method is used to convert any object in Java to String format. If we want to get object, Java Compiler automatically invokes 'toSting()' . Although we can override this method if required.

Lets Consider following example if we don't use this method.

package com.tech;
public class Employee {

	private int Salary;
	private String name;

	public Employee(String name,int Salary) {
		this.name=name;
		this.Salary=Salary;
	}
	public static void main(String[] args)
        {
		Employee e1=new Employee("Ramesh",45000);
		Employee e2=new Employee("Gaurav",25000);
		System.out.println(e1); // compiler writes here e1.toString()   
		System.out.println(e2); //// compiler writes here e2.toString()

	}

}

The Output we obtain is :

com.tech.Employee@20eb607d
com.tech.Employee@3d0bbf6d

But If we use toString() or Override this method like :

package com.tech;

public class Employee {

	private int Salary;
	private String name;

	public Employee(String name,int Salary) {
		this.name=name;
		this.Salary=Salary;
	}
	public String toString(){return name+" "+Salary;} // over ride the toString()

	public static void main(String[] args) {
		Employee e1=new Employee("Ramesh",45000);
		Employee e2=new Employee("Gaurav",25000);
		System.out.println(e1);
		System.out.println(e2);

	}

}

Now we obtain the Output as:

Ramesh 45000
Gaurav 25000