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
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
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 } } ); Q:Can I import same package/class twice? Will the JVM load the package twice at runtime? A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class. Q: What are Checked and UnChecked Exception? A: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. Q : What is Overriding? A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private. Q: What are different types of inner classes? A: Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor. Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD; Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
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