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Object Oriented Programming Articles
Page 588 of 589
Do local variables in Java have default values?
No, local variables do not have default values. Once we create a local variable we must initialize it before using it. Since the local variables in Java are stored in stack in JVM there is a chance of getting the previous value as default value. Therefore, In Java default values for local variables are not allowed. Example public class Sample { public static void main(String args[] ){ int data; System.out.println(data); } } Error C:\Sample>javac Sample.java Sample.java:4: error: variable data might not have been initialized System.out.println(data); ^ 1 error
Read MoreWhat does abstract modifier in Java do?
An abstract keyword is used to declare methods abstract methods and abstract classes. Once a method is declared abstract we should not specify body for those. Example public abstract class Sample{ public abstract demo(){ } } Error Sample.java:2: error: invalid method declaration; return type required public abstract demo(){ ^ 1 error And once a class is declared abstract it cannot be instantiated. Example public abstract class Sample{ public abstract void demo(); public static void main(String args[]){ new Sample(); } } Error C:\Sample>javac Sample.java Sample.java:4: error: Sample is abstract; cannot be instantiated new Sample(); ^ 1 error
Read MoreHow many ways a String object can be created in java?
You can create a String by − Step 1 − Assigning a string value wrapped in " " to a String type variable. String message = "Hello Welcome to Tutorialspoint"; Step 2 − Creating an object of the String class using the new keyword by passing the string value as a parameter of its constructor. String message = new String ("Hello Welcome to Tutorialspoint"); Step 3 − Passing a character array to the String constructor. char arr[] = {'H','e','l','l','o'}; String message = new String(arr);
Read MoreHow to create a string from a Java ArrayList?
To convert the contents of an ArrayList to a String, create a StringBuffer object append the contents of the ArrayList to it, finally convert the StringBuffer object to String using the toString() method. Example import java.util.ArrayList; public class String_ArrayList { public static void main(String args[]) { ArrayList al = new ArrayList(); al.add("Hello"); al.add("are"); al.add("you"); StringBuffer sb = new StringBuffer(); for (String s : al) { sb.append(s); sb.append(" "); } String str = sb.toString(); System.out.println(str); } } Output Hello are you
Read MoreWhat is the difference between compile time errors and run time errors in Java?
Compile time errors are syntactical errors in the code which hinders it from being compiled. Example public class Test{ public static void main(String args[]){ System.out.println("Hello") } } Output C:\Sample>Javac Test.java Test.java:3: error: ';' expected System.out.println("Hello") An exception (or exceptional event) is a problem that arises during the execution of a program. When an Exception occurs the normal flow of the program is disrupted and the program/Application terminates abnormally, which is not recommended, therefore, these exceptions are to be handled. Example ...
Read MoreWhat are checked exceptions in Java?
A checked exception is an exception that occurs at the time of compilation, these are also called as compile time exceptions. These exceptions cannot simply be ignored at the time of compilation; the programmer should take care of (handle) these exceptions. if you use FileReader class in your program to read data from a file, if the file specified in its constructor doesn't exist, then a FileNotFoundException occurs, and the compiler prompts the programmer to handle the exception. Example import java.io.File; import java.io.FileReader; public class FilenotFound_Demo { public static void main(String args[]) { ...
Read MoreWhat are number format exceptions in Java?
This Exception happens once you associated convert a string variable in an incorrect format to a whole number (numeric format) that's not compatible with one another. Example public class Test { public static void main(String[] args) { int data = Integer.parseInt("hello"); } } Output Exception in thread "main" java.lang.NumberFormatException: For input string: "hello" at java.lang.NumberFormatException.forInputString(Unknown Source) at java.lang.Integer.parseInt(Unknown Source) at java.lang.Integer.parseInt(Unknown Source) at a6.dateAndTime.Test.main(Test.java:5)
Read MoreWhat is the difference between System.out, System.in and System.err streams in Java?
All the programming languages provide support for standard I/O where the user's program can take input from a keyboard and then produce an output on the computer screen. Similarly, Java provides the following three standard streams:Standard Input: This is used to feed the data to user's program and usually a keyboard is used as a standard input stream and represented as System.in.Standard Output: This is used to output the data produced by the user's program and usually a computer screen is used for standard output stream and represented as System.out.Standard Error: This is used to output the error data produced ...
Read MoreHow can dead thread be restarted in Java?
A thread goes through various stages in its lifecycle. For example, a thread is born, started, runs, and then dies. New − A new thread begins its life cycle in the new state. It remains in this state until the program starts the thread. It is also referred to as a born thread. Runnable − After a newly born thread is started, the thread becomes runnable. A thread in this state is considered to be executing its task. Waiting − Sometimes, a thread transitions to the waiting state while the thread waits for another thread to perform a task. ...
Read MoreWhat is the Thread class in Java?
The java.lang.Thread class is a thread of execution in a program. The Java Virtual Machine allows an application to have multiple threads of execution running concurrently. Following are the important points about Thread −Every thread has a priority. Threads with higher priority are executed in preference to threads with lower priority.Each thread may or may not also be marked as a daemon.There are two ways to create a new thread of execution.One is to declare a class to be a subclass of Thread.Another way to create a thread is to declare a class that implements the Runnable interface.
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