Java 코딩테스트:: Java 기본 문법

1. 기본 문법

변수 선언과 초기화

int a = 10;
double b = 3.14;
char c = 'A';
String s = "Hello";

 

조건문

if (a > 5) {
    System.out.println("a is greater than 5");
} else if (a == 5) {
    System.out.println("a is 5");
} else {
    System.out.println("a is less than 5");
}

 

반복문

for (int i = 0; i < 10; i++) {
    System.out.println(i);
}

int j = 0;
while (j < 10) {
    System.out.println(j);
    j++;
}

 

 

2. 자료 구조

배열

int[] arr = new int[5];
arr[0] = 1;
arr[1] = 2;

int[] arr2 = {1, 2, 3, 4, 5};

 

 

ArrayList

import java.util.ArrayList;

ArrayList<Integer> list = new ArrayList<>();
list.add(0);
list.add(1);
list.add(2);

for (int num : list) {
    System.out.println(num);
}

 

HashMap

import java.util.HashMap;

HashMap<String, Integer> map = new HashMap<>();
map.put("one", 1);
map.put("two", 2);

for (String key : map.keySet()) {
	System.out.println(key + ": "+map.get(key));
}

 

3. 함수와 메서드

public class Main {
    public static void main(String[] args) {
        int result = add(2, 3);
        System.out.println(result);
    }

    public static int add(int a, int b) {
        return a + b;
    }
}

 

4. 클래스와 객체

public class Car {
    private String brand;
    private String model;

    // 생성자
    public Car(String brand, String model) {
        this.brand = brand;
        this.model = model;
    }

    // 메서드
    public void displayInfo() {
        System.out.println("Brand: " + brand + ", Model: " + model);
    }

    public static void main(String[] args) {
        Car car = new Car("Toyota", "Camry");
        car.displayInfo();
    }
}

 

5. 예외 처리

public class Main {
    public static void main(String[] args) {
        try {
            int result = divide(10, 0);
            System.out.println(result);
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero");
        }
    }

    public static int divide(int a, int b) {
        return a / b;
    }
}

6. 입출력

표준 입력

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
    	Scanner scanner = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = scanner.nextInt();
        System.out.println("You entered: " + num);
    }
}

파일 입출력

import java.io.*;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new FileReader("input.txt"));
        BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt"));

        String line;
        while ((line = br.readLine()) != null) {
            bw.write(line);
            bw.newLine();
        }

        br.close();
        bw.close();
    }
}

 

7. 문자열 처리

String str = "Hello, World!";
String upper = str.toUpperCase(); // "HELLO, WORLD!"
String lower = str.toLowerCase(); // "hello, world!"
String sub = str.substring(7); // "World!"
boolean contains = str.contains("World"); // true

8. 자주 사용되는 알고리즘

정렬

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] arr = {5, 3, 8, 1, 2};
        Arrays.sort(arr); // 오름차순 정렬
        System.out.println(Arrays.toString(arr));
    }
}

Arrays.sort()

Arrays.toString(arr)

 

 

탐색

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5};
        int index = Arrays.binarySearch(arr, 3); // 이진 탐색
        System.out.println("Index of 3: " + index);
    }
}

Arrays.binarySearch(arr, idx)