Core Java Interview Question Page 8
Question: What are the static fields & static Methods ?
Answer: If a field or method defined as a static, there is only one copy for entire class, rather than one copy for each instance of class. static method cannot accecss non-static field or call non-static method
Example Java Code
static int counter = 0;
A public static field or method can be accessed from outside the class using either the usual notation:
Java-class-object.field-or-method-name
or using the class name instead of the name of the class object:
Java- class-name.field-or-method-name
Question: What are the Final fields & Final Methods ?
Answer: Fields and methods can also be declared final. A final method cannot be overridden in a subclass. A final field is like a constant: once it has been given a value, it cannot be assigned to again.
Java Code
private static final int MAXATTEMPTS = 10;
Question: Describe the wrapper classes in Java ?
Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.
Following table lists the primitive types and the corresponding wrapper classes:
Primitive |
Wrapper |
boolean |
java.lang.Boolean |
byte |
java.lang.Byte |
char |
java.lang.Character |
double |
java.lang.Double |
float |
java.lang.Float |
int |
java.lang.Integer |
long |
java.lang.Long |
short |
java.lang.Short |
void |
java.lang.Void |
Question: What are different types of inner classes ?
Answer: Inner classes nest within other classes. A normal class is a direct member of a package. Inner classes, which became available with Java 1.1, are four types
- Static member classes
- Member classes
- Local classes
- Anonymous classes
Static member classes - a static member class is a static member of a class. Like any other static method, a static member class has access to all static methods of the parent, or top-level, class.
Member Classes - a member class is also defined as a member of a class. Unlike the static variety, the member class is instance specific and has access to any and all methods and members, even the parent's this reference.
Local Classes - Local Classes declared within a block of code and these classes are visible only within the block.
Anonymous Classes - These type of classes does not have any name and its like a local class
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Java Anonymous Class Example public class SomeGUI extends JFrame { ... button member declarations ... protected void buildGUI() { button1 = new JButton(); button2 = new JButton(); ... button1.addActionListener( new java.awt.event.ActionListener() <------ Anonymous Class { public void actionPerformed(java.awt.event.ActionEvent e) { // do something } } );