Java 기본 문법 정리입니다.
1. Tutorial
1.0 public class
/**
* App
*/
public class App {
public static void main(String[] args) {
// code space
}
}
1.1 print
System.out.println("Hello World");
System.out.print("this is on ");
System.out.println("same line.");
System.out.println("He said \"I\'m not ok. using Backslash (\\) \"");
// Operators
String x1 = "10";
int x2 = 20;
System.out.println(x1 + x2); // String concat operator
1.2 Data Type
// type (primitive, wrapper classes)
Byte bNull = null;
// byte b_null = null; primitive can't null
byte b1 = -128, b2 = 127; // 2^8
Byte B1 = -128, B2 = 127; // 2^8
short s1 = -32768, s2 = 32767; // 2^16
Short S1 = -32768, S2 = 32767; // 2^16
int i1 = -2147483648, i2 = 2147483647; // 2^32
Integer I1 = -2147483648, I2 = 2147483647; // 2^32
long l1 = -9223372036854775808L, l2 = 9223372036854775807L; // 2^64
Long L1 = -9223372036854775808L, L2 = 9223372036854775807L; // 2^64
float f1 = 1.23456789f; // 2^32
Float F1 = 1.23456789f; // 2^32
double d1 = 1.2345678901234567890d; // 2^64
Double D1 = 1.2345678901234567890d; // 2^64
boolean bool1 = true, bool2 = false; // 1bit
Boolean Bool1 = true, Bool2 = false; // 1bit
char c1 = 'a', c2 = 97;
Character C1 = 'b', C2 = 98;
String h1 = "hello";
String[] fruits = { "banana", "apple", "berry" };
Object[] myArray = { 1, "2" };
System.out.println(l1 - 1); // underflow
System.out.println(l2 + 1); // overflow
System.out.println(f1); // floating point error
System.out.println(d1); // floating point error
System.out.println(c1); // a
System.out.println(c2); // a
System.out.println(h1); // hello
System.out.println(fruits[2]);
1.3 Math
System.out.println(Math.E); // 2.718281828459045
System.out.println(Math.PI); // 3.141592653589793
System.out.println(Math.sin(Math.PI)); // ~ 0
System.out.println(Math.cos(Math.PI)); // -1
System.out.println(Math.tan(Math.PI / 2)); // ~ inf
System.out.println(Math.asin(0.5) / Math.PI); // 1/6
System.out.println(Math.acos(0.5) / Math.PI); // 1/3
System.out.println(Math.atan(1) / Math.PI); // 1/4
System.out.println(Math.log(Math.exp(1))); // log e = 1
System.out.println(Math.log10(Math.pow(10, 25)));// log10 10^25 = 25
System.out.println(Math.random()); // 0 ~ 1
// Exception has occurred. overflow protect
// System.out.println(Math.addExact(Integer.MAX_VALUE, 1));
System.out.println(Math.max(10, 100)); // 100
System.out.println(Math.min(10, 100)); // 10
System.out.println(Math.ceil(1234.5678)); // 1235.0 double -> double
System.out.println(Math.floor(1234.5678)); // 1234.0 double -> double
System.out.println(Math.round(1234.5678)); // 1235 float -> int, double -> long
System.out.println(Math.abs(-10)); // 10
1.4 if else, switch
boolean cond = false, cond2 = false, cond3 = false;
if (cond)
System.out.println("cond : " + cond);
else if (!cond2)
System.out.println("cond2 : not " + cond2);
else
System.out.println("cond2 : " + cond2);
System.out.println("cond3 is : " + (cond3 ? "true" : "false"));
int num = 10;
switch(num){
case 1:
System.out.println("number is 1. Specially.");
break;
default:
System.out.println("number is " + num);
break;
}
1.5 for, while
int whileNum = 5, doWhileNum = 4, forNum = 3;
int[] forArray = { 4, 2, 3, 4, 100 };
while (whileNum > 0) {
System.out.println(whileNum);
whileNum--;
}
do {
System.out.println(doWhileNum);
doWhileNum--;
} while (doWhileNum > 0);
for (int i = 0; i < forNum; i++)
System.out.println(i);
for (int forElement : forArray)
System.out.println(forElement);
2. Class
2.1 Class Attribute, Method ( + static, final)
/**
* InnerApp
*/
class InnerApp {
private Integer priAppNum = 1;
public Integer pubAppNum = 2;
public static final Integer constNum = 3;
public static void pubsMethod() {
System.out.println("this is InnerApp public static method");
}
public void pub_method() {
System.out.println("this is InnerApp public method");
}
private static void pris_method() {
System.out.println("this is InnerApp private static method");
}
private void pri_method() {
System.out.println("this is InnerApp private method");
}
// getter
public Integer getPublicAppNum() {
return pubAppNum;
}
// setter
public void setPuiblicAppNum(Integer newNum) {
if (newNum > 0)
this.pubAppNum = newNum;
else
this.pubAppNum = 0;
}
}
/**
* App
*/
public class App {
public static void main(String[] args) {
// class
InnerApp innerApp1 = new InnerApp();
// System.out.println(innerApp1.priAppNum); // private won't work
System.out.println(++innerApp1.pubAppNum);
// System.out.println(++InnerApp.constNum); // final attribute can't modified
innerApp1.pubsMethod(); // warning, but it will works
InnerApp.pubsMethod();
innerApp1.pub_method();
// InnerApp.pub_method(); // static method won't works class direct
// InnerApp.pris_method(); // private won't work
// innerApp1.pri_method(); // private won't work
innerApp1.setPuiblicAppNum(I2);
System.out.println(innerApp1.getPublicAppNum());
}
}
2.2 Constructor
/**
* ConstructorExample
*/
class ConstructorExample {
public int num1, num2;
public ConstructorExample(int _num1, int _num2) {
num1 = _num1;
this.num2 = _num2;
}
public int sumNumber() {
return (this.num1 + num2);
}
}
public class App {
public static void main(String[] args) {
ConstructorExample cExample = new ConstructorExample(2, 3);
int resultNumber = cExample.sumNumber();
System.out.println(resultNumber);
}
}
2.3 Encapsulation
public class App {
public static void main(String[] args) {
Person p1 = new Person("jjj", "qwer1234");
p1.login("jjj", "qwerqwer"); // fail
p1.login("jjj", "qwer1234"); // success
}
}
class Person {
// encapsulation
private String ID = "__unknown__";
private int PasswordHashValue = -1; // there is no password
private int secretNumArray[] = { 2, 3, 5, 7, 11 };
Person(String id, String pasword) {
this.ID = id;
this.PasswordHashValue = this.genHash(pasword);
}
// encapsulation. secret algorithm
private int genHash(String password) {
int num = 0;
char[] passwordCharArray = password.toCharArray();
for (int i = 0; i < password.length(); i++) {
num += passwordCharArray[i] * secretNumArray[i % 4];
}
return num;
}
// encapsulation
private boolean authentication(String id, String password) {
if (this.ID != id)
return false;
if (this.PasswordHashValue != this.genHash(password))
return false;
return true;
};
// public method
public void login(String id, String password) {
if (authentication(id, password)) {
System.out.println("welcome");
} else {
System.out.println("wrong information");
}
}
}
2.4 Inheritance
public class App {
public static void main(String[] args) {
Bird bird = new Bird("pp");
System.out.println(bird.name);
bird.age = 10;
System.out.println(bird.age);
bird.name = "birdy"; // protected but super(name)
System.out.println(bird.name);
bird.nickname = "new_nickname";
System.out.println(bird.nickname);
bird.eat();
bird.sleep();
bird.fly();
}
}
class Animal {
protected String name;
protected String nickname = "nickname";
public int age;
public Animal(String name) {
this.name = name;
}
public void eat() {
System.out.println(name + " is eating.");
}
public void sleep() {
System.out.println(name + " is sleeping");
}
final public void breath() {
System.out.println(name + " is breathing. all of Animal does.");
}
}
class Bird extends Animal {
public Bird(String name) {
super(name); // public Animal(String name);
}
@Override
public void eat() {
System.out.println(name + " is eating with beak");
}
// final method can't override
// @Override
// public void breath() {
// System.out.println(name + " is another breathing");
// }
public void fly() {
System.out.println(name + " flying");
}
}2.5 Polymorphism
// Compile-time Polymorphism (overloading)
public class Calculator {
public int add(int a, int b) {
return a + b;
}
public double add(double a, double b) {
return a + b;
}
}
//Runtime Polymorphism (overriding)
public class App {
public static void main(String[] args) {
Animal myDog = new Dog();
Animal myCat = new Cat();
myDog.makeSound(); // Dog barks.
myCat.makeSound(); // Cat meows.
}
}
class Animal {
public void makeSound() {
System.out.println("Animal makes a sound.");
}
}
class Dog extends Animal {
@Override
public void makeSound() {
System.out.println("Dog barks.");
}
}
class Cat extends Animal {
@Override
public void makeSound() {
System.out.println("Cat meows.");
}
}
2.6 Abstractor
abstract class Shape {
abstract double area(); // 상속할 때 반드시 구현하라는 의미. method body가 없어도 된다.
void printArea() {
System.out.println("Area : " + area());
}
}
class Circle extends Shape {
private double radius;
Circle(double radius) {
this.radius = radius;
}
@override // 없어도 error 안나지만 있으면 보기 좋다
double area() { // 없으면 에러 난다
return Math.PI * radius * radius;
}
}
2.7 Interface
public class App {
public static void main(String[] args) {
Circle circle = new Circle(5.0);
circle.printArea();
}
}
interface Shape {
double area(); // method를 만들라고 지시
}
class Circle implements Shape {
private double radius;
Circle(double radius) {
this.radius = radius;
}
@Override
public double area() {
return Math.PI * radius * radius;
}
void printArea() {
System.out.println("Area: " + area());
}
}
* reference