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super() from a static context?
A: A static variable is shared by all instances of a class. Every instance has a reference to the same variable and can modify it directly. Static instances are declared with the static modifier.
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What is special about static variables?
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A: The main method must be declared public static void because it is invoked when the Java Virtual Machine first starts up, before any Java object instances exist. The main method may reference any number of static variables and call static methods. Any series of calls to other static methods only extends this original static context. To reference a non-static or instance variable from the main method, you must first instantiate the relevant object, even if it is the host of the main method itself, as below.
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I get "non-static variable this cannot be referenced from a static context"!
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A: Calling static members in a separate class is very similar to calling a static method in a separate class. Most importantly, the static member must have a public, package or protected modifier that makes it accessible to the calling class. Use the class name, a dot separator and the name of the member variable, as with the AWT Color class properties below:
Color textColor = Color.black;
Color panelColor = Color.lightGray;
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A: The only way that a class can call a method without an object reference is in a static context. For example, the static main() method is invoked in a static context, before any instance is created, so any methods it calls must also be static. The main() method may call static methods of its own, or of other classes.
One key thing about static methods is that they must not reference any instance variables or call any instance methods because there may not be any instance of an object in the static context.
When you call a method from an object (more precisely from a reference to an instance of an object), it ensures that an instance exists and means that all instance variables are initialised and can be referenced, and instance methods can safely be called. The compiler ensures these conditions. An instance can also call a class or static method and variables of its own or other classes.
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A: To call a static method in a separate class you should prefix the method reference with the name of the class it belongs to, known as the host class. For example, the String class has valueOf() methods to convert primitive values to strings:
String booleanString = String.valueOf(true);
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A: Yes, it is possible to call a static method on an object reference using the same dot syntax as for a static class reference, as below.
String stringRef = "Example";
System.out.println(stringRef.valueOf(true));
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super() from a static context?
A: The super() method can only be called in an object's constructor, it cannot be called in a static context. The only way to invoke the super() method is to place this statement in the first line of an object's constructor and instantiate the object. You can then invoke this method via the static void main(String[]) method, as below.
public class SuperConstructor extends Superclass {
public SuperConstructor() {
super();
}
public static void main(String[] args) {
SuperConstructor object = new SuperConstructor();
}
}
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A: Static methods cannot truly be overridden, if you create static methods with the same signature as those in a superclass it will lead to confusing behaviour and is not recommended. It is not possible to use the super keyword in the static context of a subclass because it refers to a instance. Just use the standard static method reference scheme for the superclass; the superclass name, a dot separator, the method name and arguments, as below.
public class ExampleSubclass extends ExampleSuperclass {
public static void main(String[] args) {
ExampleSuperclass.testMethod();
}
}
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A: Creating a static method with the same signature as a superclass version effectively hides the superclass method, it does not override it. This is a difficult distinction to understand and to detect because in several contexts the methods behave as if they were overridden. The difference becomes apparent when an instance of the subclass is assigned to a superclass type, as below.
public class SuperclassStatic {
public static void testMethod() {
System.out.println("Superclass message");
}
}
public class SubclassStatic extends SuperclassStatic {
public static void testMethod() {
System.out.println("Subclass message");
}
public static void main(final String[] args) {
System.out.println("Calling subclass method...");
testMethod();
System.out.println("Calling superclass method...");
SuperclassStatic.testMethod();
// Subclass type
SubclassStatic subStatic = new SubclassStatic();
System.out.println("Calling static method on subclass type...");
subStatic.testMethod();
// Superclass type
SuperclassStatic superStatic = new SubclassStatic();
System.out.println("Calling static method on superclass type...");
superStatic.testMethod();
}
}
The subclass' local call to the static testMethod() executes its own version and you get the subclass message. The call to the SuperclassStatic.testMethod() executes the superclass version and you get the superclass message.
When you assign an instance of the subclass to a SubclassStatic type variable, the runtime system executes the testMethod() version associated with the variable type, so gives the subclass message. In the last case a subclass instance is assigned to a variable declared as a SuperStatic type, so the testMethod() defined in the superclass is executed and gives the superclass message.
The Java compiler does not fail or warn about static method name clashes, so it is important to be aware of this scenario and avoid it.
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static and final keywords?
A: The static and final modifiers have quite distinct purposes in Java, though they can be used in combination to declare class constants. When the static modifier is applied to a variable or method it belongs to all instances of the class and can be referenced in a static context. In other words, the class does not have to be instantiated before the variable can be read or the method called. That means that static variables must be assigned at compile time and static methods cannot reference instance variables or call instance methods.
The final modifier can be applied to a class definition or method to prevent extension or overriding respectively. This helps the compiler optimise byte code for the class. When the final modifier is declared on a variable it means that its initial assignment will not change during the execution of the code. Final instance variables must be assigned at compile time or in the class' constructor, as in the example below.
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What is the difference between static and final keywords?
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