Core Java Interview Questions- 25
1) Which Component subclass is used for drawing and painting?
Canvas
2).
3) What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or
implement the Runnable interface.
4) What methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel()
5) How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the squares of a
grid.
6).
7) What state is a thread in when it is executing?
An executing thread is in the running state.
8) What are the legal operands of the instanceof
operator?
The left operand is an object reference or null value and the right operand is a class,
interface, or array type.
9) Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.
10) What Checkbox method allows you to tell if a Checkbox is checked?
getState()
11) What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering
the data in some way as it is passed from one stream to another.
12) If an object is garbage collected, can it become
reachable again?
Once an object is garbage collected, it ceases to exist. It can no longer become reachable
again.
13) Are true and false keywords?
The values true and false are not keywords.
14) What classes of exceptions may be thrown by a throw
statement?
A throw statement may throw any expression that may be assigned to the Throwable type.
15) What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.
16) What is the difference between the File and
RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The
RandomAccessFile class provides the methods needed to directly access data contained in
any part of a file.
17) What happens when you add a double value to a String?
The result is a String object.
18) What is a void return type?
A void return type indicates that a method does not return a value.
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20) What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set.
Sets do not allow duplicate elements.
21) What is your platform’s default character encoding?
If you are running Java on English Windows platforms, it is probably Cp1252. If you are
running Java on English Solaris platforms, it is most likely 8859_1..
22) What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.
23).
24) How can a dead thread be restarted?
A dead thread cannot be restarted.
25) What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread’s
ThreadGroup being invoked, which eventually results in the termination of the program in
which it is thrown.
26) What is a layout manager?
A layout manager is an object that is used to organize components in a container.
27) Which arithmetic operations can result in the
throwing of an ArithmeticException?
Integer / and % can result in the throwing of an ArithmeticException.
28) Which package is always imported by default?
The java.lang package is always imported by default.
29) What interface must an object implement before it
can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be
written to a stream as an object.
30) How are this and super used?
this is used to refer to the current object instance. super is used to refer to the variables
and methods of the superclass of the current object instance.
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33) What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane
handles its own events and performs its own scrolling.
34) What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is
used to represent the classes and interfaces that are loaded by a Java program..
35) public and a nonpublic class?
A public class may be accessed outside of its package. A non-public class may not be
accessed outside of its package.
39) To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.
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43) What happens when you add a double value to a String?
The result is a String object.
44) What is the List interface?
The List interface provides support for ordered collections of objects.
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47) What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.
48) Can try statements be nested?
Try statements may be tested.
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50) What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement
group.
51) What are the two basic ways in which classes that can be run as threads may be defined?
A thread class may be declared as a subclass of Thread, or it may implement the Runnable
interface.
52).
53)What is an Instance in java?
Instance refers to the copy of the object at a particular time whereas object refers to the memory address of the class.
The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory.
54).What is the difference between checked and unchecked exceptions?
In general, unchecked exceptions represent defects in the program (bugs), which are normally Runtime exceptions.
Furthermore, checked exceptions represent invalid conditions in areas outside the immediate control of the program.
55).How does Java allocate stack and heap memory?
Each time an object is created in Java it goes into the area.
56).What is Java Reflection?
Reflection is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine.
57)What is an Encapsulation in Java ?
Encaps.
58)What is multiple inheritance and does java support?
If a child class inherits the property from multiple classes is known as multiple inheritance.
Java does not allow to extend multiple classes but to overcome this problem it allows to implement multiple Interfaces.
59)What is Composition?
Composition is a special type of aggregation relationship with a difference that its the compulsion for the OtherClass object to exist for the existence of MyMainClass.
60)What are the different forms of Polymorphism?
There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading.Runtime time polymorphism is done using inheritance and interface.