JAVA EE: Java Tutorial : Java pass by value (Object memory)

Wednesday 2 December 2015

Java Tutorial : Java pass by value (Object memory)

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Java Tutorial : Java pass by value (Object memory)

Employee.java

public class Employee
{

    private String name;
    private int age;

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }

    public int getAge()
    {
        return age;
    }

    public void setAge(int age)
    {
        this.age = age;
    }

}

PassReferenceDatatypeTest.java

public class PassReferenceDatatypeTest
{

    public static void main(String[] args)
    {

        Employee myEmployee = new Employee();

        System.out.println("Inside main before calling changeIt method: "
                + myEmployee);

        System.out.println("Name : " + myEmployee.getName());
        System.out.println("Age  : " + myEmployee.getAge());

        System.out.println("-------------------------- ");

        changeIt(myEmployee);

        System.out.println("Inside main after calling changeIt method: "
                + myEmployee);

        System.out.println("Name : " + myEmployee.getName());
        System.out.println("Age  : " + myEmployee.getAge());

    }

    /*
     * Passing Reference Data Type Arguments :
     * 
     * Reference data type parameters, such as objects, are also passed into
     * methods by value. This means that when the method returns, the passed-in
     * reference still references the same object as before. However, the values
     * of the object's fields can be changed in the method, if they have the
     * proper access level.
     */

    public static void changeIt(Employee employee)
    {
        employee.setName("Sawyer");
        employee.setAge(29);

        System.out.println("Inside changeIt method before assigning "
                + "new Employee object to employeeObject reference: "
                + employee);
        System.out.println("Name : " + employee.getName());
        System.out.println("Age  : " + employee.getAge());

        System.out.println("-------------------------- ");

        /*
         * Inside the method, employee initially refers to myEmployee. The method
         * changes the name and age properties of the object that employee references
         * (i.e., myEmploye) by "Sawyer" and 29, respectively. These changes will
         * persist when the method returns. Then employee is assigned a reference
         * to a new Employee object with name = null and age = 0.
         * 
         * This reassignment has no permanence, however, because the reference
         * was passed in by value and cannot change. Within the method, the
         * object pointed to by employee has changed, but, when the method
         * returns, myEmployee still references the same Employee object as
         * before the method was called.
         */
        employee = new Employee();
        System.out.println("Inside changeIt method after assigning "
                + "new Employee object to employeeObject reference: "
                + employee);
        System.out.println("Name : " + employee.getName());
        System.out.println("Age  : " + employee.getAge());

        System.out.println("-------------------------- ");

    }

}

Output

Inside main before calling changeIt method: Employee@7e0b37bc
Name : null
Age  : 0
-------------------------- 
Inside changeIt method before assigning new Employee object to employeeObject reference: Employee@7e0b37bc
Name : Sawyer
Age  : 29
-------------------------- 
Inside changeIt method after assigning new Employee object to employeeObject reference: Employee@3b95a09c
Name : null
Age  : 0
-------------------------- 
Inside main after calling changeIt method: Employee@7e0b37bc
Name : Sawyer
Age  : 29