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Java Programming
The Java programming language was released in 1995 as a core component of the Java platform of Sun Microsystems. It is a general-purpose, class-based, object-oriented language that is widely used in application software and web applications.
5,203 Questions
What is syntax errors in java?
Runtime errors in Java means that there is some code error existing in your computer and it is impossible for you to deal with it by yourself unless you are the you're the applet's designer and change the code. Most of the time, runtime errors in Java will appear error message just like:
"Java(TM) Plug-in Fatal Error
Several Java Virtual Machines running in the same process caused an error."
"Java(TM)Plug-in Fatal ERror
the Java Runtime Environment cannot be loaded."
What method of purchasing is a cost containment method?
A cost containment method for purchasing is bulk buying, where organizations purchase large quantities of goods or services at a discounted rate. This approach reduces per-unit costs, lowers inventory expenses, and can lead to better supplier negotiations. Additionally, implementing a just-in-time (JIT) purchasing strategy can minimize holding costs and reduce waste by ensuring that inventory arrives only as needed. Both methods help organizations manage expenses effectively while maintaining necessary supplies.
There are diverse ways to encrypt information (convert it to cyphertext); how a text will look in cyphertext will depend on the method chosen, as well as the specific keys.
Java does not support pointers then what is null pointer exception?
A null pointer exception is thrown when you are trying to manipulate an object that is null. It is just the name and does not have any relevance to the pointers as in C
Example:
ArrayList lst = null;
Object obj = lst.get(0);
In the first line we have declared an array list. Without initializing it we have tried to access the element in the 0th position. This would cause a null pointer exception.
What are the native methods how do you use them?
The native modifier indicates that a method is implemented in platform-dependent code (Not Java Of Course), but often in C. There wont be any questions about how to use native methods for the exam. For now it is enough if you know that native is a modifier (thus a reserved keyword) and that native can be applied only to methods-not classes, not variables, just methods. Note that a native method's body must be a semicolon (;) (like abstract methods), indicating that the implementation is omitted.
Then data will be lost. Quite often, at least in Java, the compiler will protest at compile time, basically forcing you to rethink your strategy.
What are Integer Boolean and Double wrapper classes?
In Java an integer and a double are "primitive" data types.
When dealing with functions, often you want to do more than send an "int" or a "double"'s VALE, you wish to send a REFERENCE of a number type to a function so that you can operate on that number type. What you can do is create a new Integer, Boolean, or Double, and send it to the function. This creates a "CLASS" that acts as an Integer (or whatever you chose).
Eg. You want a function that takes in a number variable, and adds two to it, and returns NOTHING. You cannot send a normal "int", because that integer cannot be "referenced" to after the function call. You would create a new Integer Object from the Integer Class, and insert that new object to the function.
Integer four = new Integer(4); // Makes a new Integer named "four" with the value 4.
There is a wrapper class for every primitive in Java. For instance, the wrapper class for int is Integer, the class for float is Float, and so on. Remember that the primitive name is simply the lowercase name of the wrapper except for char, which maps to Character, and int, which maps to Integer.
The Wrapper classes for each of the primitive types is as follows:
1. boolean - Boolean
2. byte - Byte
3. char - Character
4. double - Double
5. float - Float
6. int - Integer
7. long - Long
8. short - Short
A point to note here is that, the wrapper classes for numeric primitives have the capability to convert any valid number in string format into its corresponding primitive object. For ex: "10" can be converted into an Integer by using the below line
Integer intVal = new Integer("10");
I can do the same for all numeric types like long, short etc. But, if I pass an invalid value I will get a "NumberFormatException" Ex:
Integer intVal = new Integer("Haha");
The Wrapper Constructors
All of the wrapper classes except Character provide two constructors: one that takes a primitive of the type being constructed, and one that takes a String representation of the type being constructed-for example
Integer i1 = new Integer(42);
Integer i2 = new Integer("42");
or
Float f1 = new Float(3.14f);
Float f2 = new Float("3.14f");
The Character class provides only one constructor, which takes a char as an argument-for example
Character c1 = new Character('c');
The constructors for the Boolean wrapper take either a boolean value true or false, or a String. If the String's case-insensitive value is "true" the Boolean will be true-any other value will equate to any other value will equate to false. Until Java 5, a Boolean object couldn't be used as an expression in a boolean test-for instance
Boolean b = new Boolean("false");
if (b) // won't compile, using Java 1.4 or earlier
As of Java 5, a Boolean object can be used in a boolean test, because the compiler will automatically "unbox" the Boolean to a boolean.
A thread can be loosely defined as a separate stream of execution that takes place simultaneously with and independently of everything else that might be happening.
An applet is a piece of code that can be used for graphical user interaction. An applet generally uses java api's for applets. It could be a window/ a dialog box and some such gui.
Visit www.sun.com for examples on applets
How objects are predefind types?
An object is created from a class, like a house made from a blueprint. The object will therefore be of the type of its class. For instance, a String object will be of type String, which is defined by the String class.
a specifier tells the JVM how to treat a particular class,method and variable while executing the program. For example, final classes cannot be extended and final methods cannot be overriden and final variables cannot be changed once declared. Likewise,static methods and variables can be accessed without having to instantiate an object for their class
The Socratic method is a form of cooperative dialogue that encourages critical thinking through asking and answering questions. Named after the ancient Greek philosopher Socrates, it involves stimulating discussion by challenging assumptions and exploring ideas in depth. This method often leads participants to discover answers for themselves, promoting deeper understanding and insight. It's frequently used in educational settings to foster analytical skills and independent thought.
What is the function of keyword extern?
The extern keyword declares a variable or function and specifies that it has external linkage (its name is visible from files other than the one in which it's defined). When modifying a variable, extern specifies that the variable has static duration (it is allocated when the program begins and deallocated when the program ends). The variable or function may be defined in another source file, or later in the same file. Declarations of variables and functions at file scope are external by default. In C++, when used with a string, extern specifies that the linkage conventions of another language are being used for the declarator(s). C functions and data can be accessed only if they are previously declared as having C linkage. However, they must be defined in a separately compiled translation unit. Microsoft C++ supports the strings "C" and "C++" in the string-literal field. All of the standard include files use the extern "C" syntax to allow the run-time library functions to be used in C++ programs. If you find this info useful Please vote!!!
How do you write exponents in Java?
You can use the Math.pow function to raise one number to the power of another.
Math.pow(24, 3) will return 243
What are some sites that you can code and save java online other than Compilr.com?
You should seriously consider getting an IDE, and writing code off-line.
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There are several free IDE's, including Microsoft's Visual Studio and Sun's Eclipse.
What is the technical name for calling a base class constructor using derived class constructor?
You cannot actually call any constructor, you can only invoke construction, either by instantiating a static instance of a class or by dynamically creating one with the new operator. In the case of base class construction via a derived class, the base class constructor is invoked by the derived class' initialisation list.
Every class constructor has an initialisation list whether you define one or not (the compiler will generate one automatically if you don't). When you derive one class from another, the derived class initialisation list invokes a call to the base class default constructor. The only exception is the compiler-generated copy constructor which automatically calls the base class copy constructor.
If you define your own initialisation list, then you can explicitly invoke any base class constructor overload, thus making your construction code all the more efficient. However, copy constructors should always invoke the base class copy constructor, so if you define a copy constructor, you must explicitly invoke the base class copy constructor -- the compiler will not invoke it implicitly from a user-defined copy constructor.
While many programmer's use the constructor's body to initialise a class, this is highly inefficient. Even if you don't specify an initialisation list, one is created for you, resulting in every base class and every member variable being initialised twice, which can quickly add up to a substantial cost in performance.
The constructor's body should only really be used for initialisation when it would be difficult or impossible to do so from the initialisation list. Remember that your object doesn't physically exist until initialisation is complete, so you may not have access to some members, particularly base class members, at certain points in the initialisation process.
Initialisation must be done from the ground up, starting with the base classes and ending with the actual class members, and all in the order they were declared. Note that only direct base classes (or virtual base classes) should be invoked from the initialisation list. The base classes themselves should invoke their own base class constructors, if they have any. Thus no matter which derivative you construct, the least-derived class is always constructed first.
Why should indexes be used instead of subscripts to identify array elements?
You cannot uses indices instead of subscripts. The subscript operator [] requires an index in order to determine the subscript. Even if you don't use the subscript operator you still need an index to determine the offset of the subscript. Indeed, the only time you do not need an index is when traversing the array using a roving pointer, which is arguably more efficient than using a subscript to traverse an array since subscripts use multiplication instead of the much simpler increment/decrement operation.
Which is true about an anonymous inner class?
A lot of things are true about them. What is it that you want to know?
Compare Do-While with While Statements?
A Do-While loop looks like this:
do {
loop body
} while (condition);
and a While loop looks like this:
while (condition) {
loop body
}
The main difference is that the loop body is always run once in the Do-While loop, then the condition is checked to see if the loop should keep running. In a While loop, the condition is checked first, and it will not run the loop body at all if the condition is false.
What is high-level language compiled?
Compiling a high-level language means that the high-level source code (what is typed by the programmer) is being converted to machine code (binary, i.e. 0's and 1's) that the local computer can execute. In interpreter languages, the source code is first converted to byte code, which is either run directly (slower, but very portable), or interpreted by a virtual machine (see JVM) into machine code, and then executed (faster, but somewhat less portable). If an error is found the compilation stops and the errors are displayed.
