Java Crash Course: Member Access

Member access

Follow along with the instructions at: https://fullstackclouddeveloper.com/course/java-for-beginners/index.html#member_access

Class level access

In Java, a class may be declared as public which means it is visible to all classes anywhere. If the class is not declared as public it is assigned default access which means it is only visible to classes within the same package.

A class with public access

package com.mypackage;

public class MyClass {
public String sayHello() {
return "hello";
}
}

Some other class in another package or another application has access to MyClass.

package com.someotherpackage;

import com.mypackage.MyClass;

public class SomeOtherClass {
public void hasAccessToMyClass() {
System.out.println(new MyClass().sayHello());
}
}

A class with default access

In this example, MyClass, with default access, can only be accessed by classes within the same package.

package com.mypackage;

class MyClass {
public String sayHello() {
return "hello";
}
}

Some other class in another package or another application does not have access to MyClass.

package com.someotherpackage;

import com.mypackage.MyClass; // error

public class SomeOtherClass {
public void doesNotHaveAccessToMyClass() {
MyClass mine = new MyClass(); // error
}
}

Member level access

public

A class member variable or function defined as public will be visible to all classes anywhere.

package com.mypackage;

public class MyClass {

public String name = "John";

public String sayHello() {
return "hello";
}
}

Some other class in another package or another application has access to name and sayHello.

package com.someotherpackage;

import com.mypackage.MyClass;

public class SomeOtherClass {
public void hasAccessToSayHelloAndName() {
MyClass myClass = new MyClass();
System.out.println(myClass.sayHello());
System.out.println(myClass.name);
}
}
protected
A class member variable or function defined as protected will only be visible to subclasses and classes within the same package.

package com.mypackage;

public class MyClass {

protected String name = "John";

protected String sayHello() {
return "hello";
}
}

Some other class in another package or another application does not have access to MyClass name or sayHello.

package com.someotherpackage;

import com.mypackage.MyClass;

public class SomeOtherClass {
public void hasNoAccessToSayHelloAndName() {
MyClass myClass = new MyClass();
System.out.println(myClass.sayHello()); // error
System.out.println(myClass.name); // error
}
}

default

A class member variable or function defined as default (no declaration) will only be visible to classes within the same package.

package com.mypackage;

public class MyClass {

String name = "John";

String sayHello() {
return "hello";
}
}

Some other class in another package or another application does not have access to MyClass name or sayHello.

package com.someotherpackage;

import com.mypackage.MyClass;

public class SomeOtherClass {
public void hasNoAccessToSayHelloAndName() {
MyClass myClass = new MyClass();
System.out.println(myClass.sayHello()); // error
System.out.println(myClass.name); // error
}
}

private

A class member variable or function defined as private will only be visible to other members within the same class.

package com.mypackage;

public class MyClass {

private String name = "John";

private String sayHello() {
return "hello";
}

public String canAccessSayHelloAndName() {
return sayHello() + " " + name;
}
}

Some other class in another package or another application does not have access to MyClass name or sayHello.

package com.someotherpackage;

import com.mypackage.MyClass;

public class SomeOtherClass {
public void hasNoAccessToSayHelloAndName() {
MyClass myClass = new MyClass();
System.out.println(myClass.sayHello()); // error
System.out.println(myClass.name); // error
}
}