'Back-End/Java_1'에 해당되는 글 48건

Input by Scanner

- The Java Scanner class breaks the input into tokens using a delimiter that is whitespace by default.

- It provides many methods to read and parse various primitive values

- Java Scanner class is widely used to parse text for string and primitive types using regular expression

1. To get input from console

public class Scan { public static void main(String[]args){ Scanner sc = new Scanner(System.in); System.out.println("Name"); String name = sc.next(); System.out.println("No"); int no = sc.nextInt(); System.out.println(name +""+no); sc.close(); } }

2. With delimiter

public class Scan { public static void main(String[]args){ String input = "hello world 10"; Scanner s = new Scanner(input).useDelimiter("\\s"); System.out.println(s.next()); System.out.println(s.next()); System.out.println(s.nextInt()); s.close(); } }  

This keyword

1. Usage of java this keyword

1) this keyword can be used to refer current class instance variable

2) this() can be used to invoke current class constructor

3) this keyword can be used to invoke current class method

4) this can be passed as an argument in the method call

5) this can be passed as an argument in the constructor call

6) this keyword can also be used to return the current class instance

2. Without this keyword

public class Test { int id; String name; Test(int id, String name){ id = id; name = name; } void display(){ System.out.println(id + name); } public static void main(String[]args){ Test a = new Test(111,"kyle"); Test b = new Test(222,"tom"); a.display(); b.display(); } }

- We use a this key word to distinguish between local variable and instance variable

- we have a this 

Test(int id, String name){ this.id = id; this.name = name; }  

- If local variables and instance variables are different, there is no need to use this keyword

Static_2

- The static keyword in java is used for memory management mainly

- It makes your program memory efficient

1) Static variable

- The static variable can be used to refer the common property of all objects 

- The static variable gets memory only once in class area at the time of class loading

1_1. Understanding problem without static variable

class Student{ int number; String name; String college = "HV"; }

- Suppose there are 500 students in my college

- All instance data members will get memory each time when object is created

- If we make it static, college field will get memory only once

1_2. Example

package statics; public class Student { int number; String name; static String college = "HV"; Student(int r, String n){ this.name = n; this.number = r; } void display(){ System.out.println(number+" "+name+" "+college); } public static void main(String[]args){ Student aa = new Student(111,"Kyle"); } }  

1_3. Program Counter

1) Without static variable

package statics; public class Counter { int count = 0; Counter(){ count++; System.out.println(count); } public static void main(String[] args){ Counter c1 = new Counter(); Counter c2 = new Counter(); } }  

- Since instance variable gets the memory at the time of object creation, each object will have the copy of the instance variable

- If it is incremented, it won't reflect to other objects

2) static variable

package statics; public class Counter { static int count = 0; Counter(){ count++; System.out.println(count); } public static void main(String[] args){ Counter c1 = new Counter(); Counter c2 = new Counter(); } }  

2) Static method

- A static method belongs to the class rather than object of class

- A static method can be invoked without the need for creating an instance of a class

- static method can access static data member and can change the value of it

2_1. Example

1) change

package statics; public class Student { int number; String name; static String college = "HV"; static void change(){ college = "BB"; } Student(int r, String n){ this.name = n; this.number = r; } void display(){ System.out.println(number+" "+name+" "+college); } public static void main(String[]args){ Student.change(); Student aa = new Student(111,"Kyle"); Student bb = new Student(222,"Tom"); aa.display(); bb.display(); } }  

2) Calculate

class Calculate{ static int cube(int x){ return x*x*x; } public static void main(String args[]){ int result=Calculate.cube(5); System.out.println(result); } }  

3) Restriction

- The static method can not use non static data member or call non-static method directly

- this and super cannot be used in static context

package statics; public class Student { int a = 40; public static void main(String[] args){ System.out.println(a); } }  

 4) Why java main method is static?

- Object is not required to call static method

- If it were non-static method, jvm create object first then call main() that will lead the problem of extra memory allocation

5) Java static block

- Is used to initialize the static data member

- It it executed before main method at the time of classloading

class A2{ static{System.out.println("static block is invoked");} public static void main(String args[]){ System.out.println("Hello main"); } }  

6) Can we execute a program without main() method?

- One of the way is static block but not in JDK 1.7

class A3{ static{ System.out.println("static block is invoked"); System.exit(0); } }  

Constructor_2

- Constructor is a special type of method that is used to initialize the object

- Constructor is invoked at the time of object creation

1. Rule

1) Constructor name must be same as its class name

2) Constructor must have no explicit return type

2. Type of constructor

1) Default constructor

2) Parameterized constructor

3. Default Constructor

3_1. Example

public class DefaultConstructor { public DefaultConstructor(){ System.out.println("Default"); } public static void main(String a[]){ DefaultConstructor defaultConstructor = new DefaultConstructor(); } }

- If there is no constructor in a class, compiler automatically creates a default constructor

3_2. What is the purpose of default constructor?

- Default constructor provides the default values to the object like 0, null, etc

package constructor; public class DefaultConstructor { int id; String name; void display(){ System.out.println(id+" "+name); } public static void main(String a[]){ DefaultConstructor defaultConstructor = new DefaultConstructor(); defaultConstructor.display(); } }  

4. Parameterized Constructor

4_1. Why use parameterized constructor?

- Parameterized constructor is used to provide different values to the distinct objects

package constructor; public class Overloading { public Overloading(int i, String j){ System.out.println("Inside double paramter constructor"); } public static void main(String a[]){ Overloading doubleV = new Overloading(1,"test"); } }  

5. Copy Constructor

- There are many ways to copy the values of one object into another in java

1) By constructor

2) By assigning the values of one object into another

3) By clone() method of Object class

5_1. Example

package constructor; public class copy { int id; String name; public copy(int i, String n){ id = i; name = n; } public copy(copy c){ id=c.id; name=c.name; } void display(){ System.out.println(id+" "+name); } public static void main(String[]args){ copy s1 = new copy(111,"kyle"); copy s2 = new copy(s1); s2.display(); } }