Basics Flashcards
class?
- A class is a user-defined type that describes what a certain type of object will look like.
- An object is an instance of a class.
public static void main() ?
- Public: It is an Access modifier, which specifies from where and who can access the method. Making the main() method public makes it globally available. It is made public so that JVM can invoke it from outside the class as it is not present in the current class.
- Static: It is a keyword which is when associated with a method, makes it a class related method. The main() method is static so that JVM can invoke it without instantiating the class. This also saves the unnecessary wastage of memory which would have been used by the object declared only for calling the main() method by the JVM.
- Void: It is a keyword and used to specify that a method doesn’t return anything. As main() method doesn’t return anything, its return type is void. As soon as the main() method terminates, the java program terminates too. Hence, it doesn’t make any sense to return from main() method as JVM can’t do anything with the return value of it.
- main: It is the name of Java main method. It is the identifier that the JVM looks for as the starting point of the java program. It’s not a keyword.
- String[] args: It stores Java command line arguments and is an array of type java.lang.String class. Here, the name of the String array is args but it is not fixed and user can use any name in place of it.
- Why is Java not a pure object oriented language?
Java supports primitive data types - byte, boolean, char, short, int, float, long, and double and hence it is not a pure object-oriented language.
- Pointers are used in C/ C++. Why does Java not make use of pointers?
Pointers are quite complicated and unsafe to use by beginner programmers. Java focuses on code simplicity, and the usage of pointers can make it challenging. Pointer utilization can also cause potential errors. Moreover, security is also compromised if pointers are used because the users can directly access memory with the help of pointers.
Thus, a certain level of abstraction is furnished by not including pointers in Java. Moreover, the usage of pointers can make the procedure of garbage collection quite slow and erroneous. Java makes use of references as these cannot be manipulated, unlike pointers.
- What do you understand by an instance variable and a local variable?
1.Instance variables: are those variables that are accessible by all the methods in the class. They are declared outside the methods and inside the class. These variables describe the properties of an object and remain bound to it at any cost.
All the objects of the class will have their copy of the variables for utilization. If any modification is done on these variables, then only that instance will be impacted by it, and all other class instances continue to remain unaffected. 2.Local variables: are those variables present within a block, function, or constructor and can be accessed only inside them. The utilization of the variable is restricted to the block scope. Whenever a local variable is declared inside a method, the other class methods don’t have any knowledge about the local variable.
- What do you mean by data encapsulation?
Data Encapsulation is an Object-Oriented Programming concept of hiding the data attributes and their behaviors in a single unit.
It helps developers to follow modularity while developing software by ensuring that each object is independent of other objects by having its own methods, attributes, and functionalities.
It is used for the security of the private properties of an object and hence serves the purpose of data hiding.
- Tell us something about JIT compiler.
JIT stands for Just-In-Time and it is used for improving the performance during run time. It does the task of compiling parts of byte code having similar functionality at the same time thereby reducing the amount of compilation time for the code to run. The compiler is nothing but a translator of source code to machine-executable code. But what is special about the JIT compiler? Let us see how it works: First, the Java source code (.java) conversion to byte code (.class) occurs with the help of the javac compiler. Then, the .class files are loaded at run time by JVM and with the help of an interpreter, these are converted to machine understandable code. JIT compiler is a part of JVM. When the JIT compiler is enabled, the JVM analyzes the method calls in the .class files and compiles them to get more efficient and native code. It also ensures that the prioritized method calls are optimized. Once the above step is done, the JVM executes the optimized code directly instead of interpreting the code again. This increases the performance and speed of the execution.
Features of java ?
- Object Oriented
- Platform Independent
- Robust
- Interpreted
- Object Oriented
In Java, everything is an Object. Java can be easily extended since it is based on the Object model.
- Why is Java a platform independent language?
Java language was developed in such a way that it does not depend on any hardware or software due to the fact that the compiler compiles the code and then converts it to platform-independent byte code which can be run on multiple systems.
The only condition to run that byte code is for the machine to have a runtime environment (JRE) installed in it.
- Robust?
Java makes an effort to eliminate error prone situations by emphasizing
mainly on compile time error checking and runtime checking.
- Java is Interpreted ?
Java byte code is translated on the fly to native machine
instructions and is not stored anywhere. The development process is more rapid
and analytical since the linking is an incremental and light-weight process.
Example prgm to print hello in java?
public class MyFirstJavaProgram { public static void main(String []args) { System.out.println("Hello World"); } }
Save, compile, & run -java program ?
Save the file as: filename.java.
C:> javac MyFirstJavaProgram.java
C:> java MyFirstJavaProgram
Hello World
enums ?
- Enums were introduced in Java 5.0. Enums restrict a variable to have one of only a few predefined values.
- The values in this enumerated list are called enums.
With the use of enums it is possible to reduce the number of bugs in your code. - For example, if we consider an application for a fresh juice shop, it would be possible to restrict the glass size to small, medium, and large. This would make sure that it would not allow anyone to order any size other than small, medium, or large.
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