Java Interview Questions - Core Faq
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Java application runs the same bytecodes regardless of any environment (Operating System). To enable a Java application to execute anywhere on the network, the compiler generates an architecture-neutral object file format. An architecture-neutral object file format meaning that compiled Java code (bytecode) can run on many processors given the presence of a JVM. The JVM is the main component of making the java a platform independent language. That is the architectural neutral part. More about.... Java Virtual Machine
No, JDK (Java Development Kit) isn't required on each machine to run a Java program. Only JRE is required, it is an implementation of the Java Virtual machine (JVM), which actually executes Java programs. JDK is development Kit of Java and is required for development only. It is a bundle of software components that is used to develop Java based applications.
Packages are used to organizes a set of related classes and interfaces. Programs are organized as sets of packages. Each package has its own set of names for types, which helps to prevent name conflicts. Package constructs can be mapped to a file system.
can be replaced:
Package cannot be nested. One source file can only have one package statement.
No, you don't need to import java.lang.*; All classes in the java.lang package are imported by default and it includes the core Java language classes.
Yes, there are many options available to make a Java program to a .exe file like Executable Jar File, JSmooth, JexePack, LaunchAnywhere etc. But there is a simple way, to make a .bat with the following code:
then convert the bat to an exe using any .bat to .exe converter.
There are five type of memory allocated by JVM
- Class(Method) Area
- Program Counter Register
- Native Method Stack
Java is always pass-by-value, not reference. Java does manipulate objects by reference, and all object variables are references. So Objects, Arrays, Primitive Types etc. – these are all passed by value in Java.
Pass by Value: When a parameter is passed by value, the caller and callee have two independent variables with the same value. If the callee modifies the parameter variable, the effect is not visible to the caller.
Pass by Reference: When a parameter is passed by reference, the caller and the callee use the same variable for the parameter. If the callee modifies the parameter variable, the effect is visible to the caller's variable.
Primitive types are the most basic data types available within the Java language. It is predefined by the language and is named by a reserved keyword. The eight primitive data types supported by the Java programming language are: boolean, char, byte, short, int, long, float and double.
- boolean, the type whose values are either true or false
- char, the character type whose values are 16-bit Unicode characters
No, null is not a keyword. The null is a literal similar to true and false in java, but it is a character string that is treated specially by the compiler if the compiler encounters it in a java source file. It treated as a reserved word in java language.
No, true, false, and null are not Java keywords, they are literals. Literals are something which are actual values not the variable names. If you try to use them as variables you will get compile error saying - invalid VariableDeclaratorId. They treated as reserved words in java language and you cannot use them as identifiers in your programs.
A JAR file is actually just a ZIP file, typically used to aggregate many Java class files and associated metadata and resources (text, images, etc.) into one compressed file for distribution.
The jar tool provides many switches, some of them are as follows:
-c creates new archive file.
-v generates verbose output.
-m includes manifest information from the given mf file.
-f specifies the archive file name.
-x extracts files from the archive file.
In case, if you don't have a manifest in your jar file the above method will not work.
Try this command if you don't have a manifest:
There are two data types available in Java -
- Primitive Data Types
- Reference/Object Data Types
Type Casting really means that, taking an Object of one particular type and "turning it int" another Object type. This process is called casting a variable.
In the above case, the object being stored in 'obj' is actually a string, and therefore we can cast to a string without any problems.
There are two ways casting could go wrong. Firstly, if you are casting between two types in completely different inheritance hierarchies then the compiler will fail and stop you:
In the above casting compilation will fail.
Secondly, if they are in the same hierarchy but still an invalid cast then a ClassCastException will be thrown at runtime.
In the above case ClassCastException will thrown.
More about.... Type Casting in Java
Up-casting is casting to a supertype, while Downcasting is casting to a subtype. Supercasting is always allowed, but subcasting involves a type check and can throw a ClassCastException. Generally, upcasting is not necessary. Casting a subtype object into a supertype is called upcast. In Java, we need not add an explicit cast and you can assign the object directly. Compiler will understand and cast the value to supertype. By doing this, we are lifting an object to a generic level. If we prefer, we can add an explicit cast and no issues.
Here, the feed() method can accept any object which is subtype of Animal. So objects of type Dog and Cat will be upcasted to Animal when they are passed into this feed() method:
More about.... Type Casting in Java
Downcasting is casting to a subtype , while Up-casting is casting to a supertype. Supercasting is always allowed, but subcasting involves a type check and can throw a ClassCastException. By doing Downcasting, we are telling the compiler that the value stored in the base object is of a super type. Then we are asking the runtime to assign the value. Because of downcast we get access to methods of the subtype on that object. When performing downcasting, that you’re well aware of the type of object you’ll be casting.
Here, we cast the Animal type to the Dog type. As Dog is subclass of Animal, this casting is called downcasting.
Downcasting can fail if the actual object type is not the target object type.
The above code will throw a ClassCastException because the actual object type is Cat. And a Cat is not a Dog so we cannot cast it to a Dog.
More about.... Type Casting in Java
No, Java will not implicitly narrow a primitive value. You cannot cast from a double to byte because the byte has a range smaller than the double and it does not contain decimals like a double does. You can explicitly cast it to a Byte via a boxing conversion. This guards against accidental loss of precision.
When you cast the double to a byte (lowercase) explicitly; then Java will implicitly box the byte into a Byte.
Because floats and doubles cannot accurately represent the base 10 multiples we use for money, so it is impossible to represent 0.1 (or any other negative power of ten). This issue is not only in Java, it's for any programming language that uses native floating-point types, as it stems from how computers handle floating-point numbers by default.
ExampleSuppose you have $1.03 and you spend 42c. Calculate how much money remaining you?
It prints 0.6100000000000001.
The solution to this problem is to use BigDecimal, int or long for monetary calculations.
The default value for a boolean (primitive) is false.
The default value for a Boolean (object) is null.
The local variables are not initialized to any default values.
Normally, local variables are declared to do some calculation. So its the developer's decision to set the value of the variable and it should not take any default value. If the developer, by mistake, did not initialize a local variable and it take default value, the java compiler throws error. So in case of local variables, the compiler will ask the developer to initialize with some value before they access the variable to avoid the usage of undefined values.
- Set of Constant declaration
- Restrict input parameter in method
- Can be usable in switch-case
- Type safety and value safety
No, Enum types are final by design. An enum cannot extends another class because an enum already extends Enum < T > . That class provides all the enum functionality. All enums implicitly extend java.lang.Enum. Because a class can only extend one parent and the Java language does not support multiple inheritance of state, therefore an enum cannot extend anything else. More about.... Enum in Java
The main() method is the starting point of an application. When a program starts running, it has to start execution from somewhere. That somewhere is called main. Without it, there is no place to start running a Java program.
Every Java application must contain a main method whose signature looks like this:
How the main Method Gets Called?
When the Java interpreter executes an application, it starts by calling the class's main() method. The main method then calls all the other methods required to run your application.
If you try to invoke the Java interpreter on a class that does not have a main method, the interpreter refuses to compile your program and displays an error message similar to this:
The main method is static because it keeps things simpler. Since the main method is static JVM (Java virtual Machine) can call it without creating any instance of a class which contains the main method.
The main() method must be declared public, static, and void. It must accept a single argument that is an array of strings. This method can be declared as either:
Prior to Java 7, you can compile and execute without main() method By using static block.
The System.exit(0) lets the program exit before the Java Virtual Machine is looking for the main() method, otherwise the following error will be thrown:
No, the main method is the default entry point for a programme. That doesn't mean that, every java class should contain main method. It is driver program which handles all other java files. It is important to note that all classes of java should be connected with main() method directly or indirectly.
The method names comes last (main), the return type the second to last (void), and then after that it's your choice.
No. The main method's return type must be void, because the java language specification enforces it. You can't change the return type of main(). Once the main is finished the program is dead, so you don't have any benefit from that.
Up to Java 6 it was possible to do this using the Static Initialization Block.
Sequence is as follows:
- jvm loads class
- executes static blocks
- looks for main method and invokes it
From Java 7, however, this does not work anymore, even though it compiles, the following error will appear when you try to execute it:
No. In Java, Overriding simply means that the particular method would be called based on the run time type of the object and not on the compile time type of it. Overriding depends on having an instance of a class. The point of polymorphism is that you can subclass a class and the objects implementing those subclasses will have different behaviours for the same methods defined in the superclass (and overridden in the subclasses). So, static methods also cannot be overridden, because static methods are a part of the Class itself, and not
a part of any object(instance) of that class. A static method is not associated with any instance of a class so the concept is not applicable.
Overloading : Yes, static methods can be overloaded just like any other method.
A static method do not have direct access to the non-static members. It needs an instance of the class in order to get access to
the non-static variable.
A static block cannot return any value from static block and keywords "this", "super" cannot be used.
Yes, you can call a non-static method from a static method is to have an instance of the class containing the non-static method.
No, you cannot override private method. A private method cannot be overridden because it is not accessible to the derived class.
- Important features of Java
- What is the difference between JDK and JRE?
- What gives Java its 'write once and run anywhere' nature?
- What is JVM and is it platform independent?
- What is Just-In-Time (JIT) compiler?
- What is the garbage collector in Java?
- What is NullPointerException in Java
- Difference between Stack and Heap memory in Java
- How to set the maximum memory usage for JVM?
- What is numeric promotion?
- Why do we need Generic Types in Java?
- What does it mean to be static in Java?
- What are final variables in Java?