How to create inner classes in Java?
In Java, to create an object for a class we use new keyword. The new keyword creates an object of a class and initializes the object by calling it’s constructor. It is a strait forward thing to create an object for a class. But, if we have another class within the class, then how we create an instance of inner class?
Can you make a class static in Java?
The answer is Yes, some classes can be made static in Java. Java supports Static Instance Variables, Static Methods, Static Block and Static Classes. Java allows a class to be defined within another class. These are called Nested Classes. The class in which the nested class is defined is known as the Outer Class. Unlike top level classes, Inner classes can be Static.
Why there is no static class in Java?
The static can be:
- variable (also known as class variable)
- method (also known as class method)
- block
- nested class
Why do we need static class in Java?
- Static nested class is static member and can be accessed like any other static member.
- Static nested classes can be imported using static imports in Java.
- Static nested classes should always be preferred as explained.
Can an inner class be declared static?
Nested classes that are declared static are simply called static nested classes. Non-static nested classes are called inner classes. For example, to create an object for the static nested class, use this syntax: OuterClass.
Is inner class should be static?
So the inner class must either itself be static (in which case no owning instance is required), or you only create the inner class instance from within a non-static context.
When can an inner class be static?
If the nested class does not access any of the variables of the enclosing class, it can be made static. The advantage of this is that you do not need an enclosing instance of the outer class to use the nested class.
Why we make inner class as static in Java?
It can directly access static members (instance field and methods) of enclosing class same like the procedural style of getting value without creating object. Static inner class can declare both static and non-static members. The static methods have access to static members of main class.
Is static inner class thread safe?
We know static variables are not thread safe.
What is the difference between an inner class and a static nested class?
1) First and most important difference between Inner class and nested static class is that Inner class require instance of outer class for initialization and they are always associated with instance of enclosing class. On the other hand nested static class is not associated with any instance of enclosing class.
Why inner classes Cannot have static declarations?
Quoting another answer, It's because an inner class is implicitly associated with an instance of its outer class, it cannot define any static methods itself.
Can inner class be protected in Java?
Java inner class is defined inside the body of another class. Java inner class can be declared private, public, protected, or with default access whereas an outer class can have only public or default access.
How do you create a static inner class object in Java?
Java static nested class example with instance methodclass TestOuter1{static int data=30;static class Inner{void msg(){System.out.println("data is "+data);}}public static void main(String args[]){TestOuter1.Inner obj=new TestOuter1.Inner();obj.msg();More items...
Can we declare local inner class as abstract?
Local Inner classes are not a member of any enclosing classes. They belong to the block they are defined within, due to which local inner classes cannot have any access modifiers associated with them. However, they can be marked as final or abstract.
Can we declare outer class as static in Java?
We can't declare outer (top level) class as static because the static keyword is meant for providing memory and executing logic without creating Objects, a class does not have a value logic directly, so the static keyword is not allowed for outer class.
What does non static inner class mean?
The instance of non static inner class is created with the reference of object of outer class, in which it has defined, this means it has enclosing instance. But the instance of static inner class is created without the reference of Outer class, which means it does not have enclosing instance. See this example.
What is the difference between static inner and non static inner?
There are two differences between static inner and non static inner classes. In case of declaring member fields and methods, non static inner class cannot have static fields and methods. But, in case of static inner class, can have static and non static fields and method. The instance of non static inner class is created with the reference ...
What are the two cases of static inner class?
If main class in accessing members of static inner class it has two cases: Case 1: For static members, it can use class name of static inner class. Case 2: For non-static members, it can create instance of static inner class. Share.
What is the difference between a static and a nested class?
10. Discussing nested classes... The difference is that a nested class declaration that is also static can be instantiated outside of the enclosing class. When you have a nested class declaration that is not static, Java won't let you instantiate it except via the enclosing class.
What is a non-static nested class?
A non-static nested class is indeed an inner class, along with anonymous classes and local classes. Each instance of a non-static nested class is implicitly associated with an enclosing instance of its containing class...
Can a static nested class invoke non-static methods?
A static nested class does not have a reference to a nesting instance, so a static nested class cannot invoke non-static methods or access non-static fields of an instance of the class within which it is nested. Let's look in the source of wisdom for such questions: Joshua Bloch's Effective Java:
Can non static inner class declare static methods?
Non-static inner class cannot declare static field and static methods. It has to be declared in either static or top level types. You will get this error on doing so saying "static fields only be declared in static or top level types".
Java Inner Classes
In Java, it is also possible to nest classes (a class within a class). The purpose of nested classes is to group classes that belong together, which makes your code more readable and maintainable.
Private Inner Class
Unlike a "regular" class, an inner class can be private or protected. If you don't want outside objects to access the inner class, declare the class as private:
Static Inner Class
An inner class can also be static, which means that you can access it without creating an object of the outer class:
Access Outer Class From Inner Class
One advantage of inner classes, is that they can access attributes and methods of the outer class:
How to instantiate inner class?
To instantiate the inner class, initially you have to instantiate the outer class . Thereafter, using the object of the outer class, following is the way in which you can instantiate the inner class. The following program shows how to access the private members of a class using inner class.
What is inner class?
As mentioned earlier, inner classes are also used to access the private members of a class. Suppose, a class is having private members to access them. Write an inner class in it, return the private members from a method within the inner class, say, getValue (), and finally from another class (from which you want to access the private members) ...
What is an anonymous inner class?
Anonymous Inner Class. An inner class declared without a class name is known as an anonymous inner class. In case of anonymous inner classes, we declare and instantiate them at the same time. Generally, they are used whenever you need to override the method of a class or an interface.
What is a nested class in Java?
Writing a class within another is allowed in Java. The class written within is called the nested class, and the class that holds the inner class is called the outer class.
Can you write a class within a method in Java?
In Java, we can write a class within a method and this will be a local type. Like local variables, the scope of the inner class is restricted within the method.
Is outer demo inner or outer?
Here you can observe that Outer_Demo is the outer class, Inner_Demo is the inner class, display_Inner () is the method inside which we are instantiating the inner class, and this method is invoked from the main method.
Can you declare an inner class private?
You just need to write a class within a class. Unlike a class, an inner class can be private and once you declare an inner class private, it cannot be accessed from an object outside the class. Following is the program to create an inner class and access it.
Why are inner classes important?
Therefore, an inner class instance can access all of the members of its outer class, without using a reference to the outer class instance. For this reason inner classes can help make programs simple and concise.
What is a nested class in Java?
Java allows a class to be defined within another class. These are called Nested Classes. The class in which the nested class is defined is known as the Outer Class. Unlike top level classes, Inner classes can be Static. Non-static nested classes are also known as Inner classes. An instance of an inner class cannot be created without an instance ...
Can a static nested class be instantiated?
A static nested class may be instantiated without instantiating its outer class. Inner classes can access both static and non-static members of the outer class. A static class can access only the static members of the outer class. class OuterClass {. private static String msg = "GeeksForGeeks"; public static class NestedStaticClass {.
What is static nested class?
Static Nested Classes: Static Nested Classes are the static classes in Java which are created inside another outer class. As these are declared inside another class with keyword ‘static’, therefore they are treated like the static member of its outer class.
What is inner class?
These are also called as Inner Classes. These classes do not allow static variables and methods in it as it is bound to the object of the outer class and exist only if the object of the outer class is created. The use of inner classes makes the code more readable and maintainable as it is used to logically group classes in one place.
What is nested in Java?
In above example, Nested is the static nested class which is declared inside Outer class. To access the static Nested Class, we need to create the object of that class through: Here, Outer.Nested is used because Nested is the static class inside Outer class.
Why is outer.nested used?
Here, Outer.Nested is used because Nested is the static class inside Outer class. As static variables can’t be accessed with the object of outer class, therefore, we need to access it using dot operator along with Outer class name.
Why is variable a not directly accessible in a nested class?
Here, a is not directly accessible in nested class because it is an instance variable of Outer class so cannot be used without the object of Outer class. It can be possible through: Variable b is accessible directly as it is a static variable and is not bound to the object of Outer class.
Can an outer class have more than one instance?
A single instance of the outer class can be associated with more than one instances of the inner class. The keyword ”this” can be used in the non-static method to refer to the current object. To access the outer class instance inside the method show (), we write Outer.this.
Does JVM know which class is nested?
JVM does not know about which class is the nested class of which class. For example, if the outer class name is Outer and nested class name is Nested then Outer$Nested.class file gets created automatically. ‘Static’ keyword in static Nested Classes does not mean that it will be loaded in class area. JVM treats it just like another class.
