ソフトウェアエンジニアの技術ブログ：Software engineer tech blog

ArrayList

import java.util.*;

public class MyApp {

	public static void main(String[] args){

		List<Integer> sales = new ArrayList<>();

		sales.add(10);
		sales.add(20);
		sales.add(30);

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

		sales.set(0, 100);
		sales.remove(2);

		for (Integer sale: sales){
			System.out.println(sale);
		}
	}
}

HashSet // 重複しない、順番が定まらない
L TreeSet, LinkedHashSet

public static void main(String[] args){

		// HashSet<Integer> = new HashSet<>();
		Set<Integer> sales = new HashSet<>();

		sales.add(10);
		sales.add(20);
		sales.add(30);
		sales.add(10);

		System.out.println(sales.size());
		for(Integer sale:sales){
			System.out.println(sale);
		}

		sales.remove(30);

		for(Integer sale:sales){
			System.out.println(sale);
		}
	}

HashMap

public class MyApp {

	public static void main(String[] args){
		// HashMap: key value
		// HashMap<String, Integer> sales = new HashMap<>();
		Map<String, Integer> sales = new HashMap<>();

		sales.put("tom", 10);
		sales.put("bob", 20);
		sales.put("steve", 30);

		System.out.println(sales.get("tom"));
		System.out.println(sales.size());

		for(Map.Entry<String, Integer> sale: sales.entrySet()){
			System.out.println(sale.getKey() + ":" + sale.getValue());
		}

		sales.put("tom", 100);
		sales.remove("steve");

		for(Map.Entry<String, Integer> sale: sales.entrySet()){
			System.out.println(sale.getKey() + ":" + sale.getValue());
		}
	}
}

StreamAPI

public static void main(String[] args){
		List<Integer> sales = new ArrayList<>(Arrays.asList(12, 30, 22, 4, 9));

		// for(Integer sale: sales){
		// 	System.out.println(sale);
		// }
		sales
			.stream()
			.filter(e-> e % 3 == 0)
			.map(e-> "(" + e + ")")
			.forEach(System.out::println);
		
	}

LocalDateTime class

import java.time.*;
import java.time.format.DateTimeFormatter;

public class MyApp {

	public static void main(String[] args){
		LocalDateTime d = LocalDateTime.now();

		System.out.println(d.getYear());
		System.out.println(d.getMonth().getValue());

		System.out.println(d.plusMonths(2).minusDays(3));

		DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy!MM!dd!");
		System.out.println(d.format(dtf));
	}
}

exception

class MyException extends Exception {
	public MyException(String s){
		super(s);
	}
}

public class MyApp {

	public static void div(int a, int b){
		try {
			if(b < 0){
				throw new MyException("not minus!");
			}
			System.out.println(a / b);
		} catch(ArithmeticException e){
			System.err.println(e.getMessage());
		} catch(MyException e){
			System.err.println(e.getMessage());
		} finally {
			System.out.println(" -- end -- ");
		}
	}

	public static void main(String[] args){
		div(3, 0);
		div(5, -2);
	}
}

wrapper class

public class MyApp {

	public static void main(String[] args){
		// Integer i = new Integer(32);
		// int n = i.intValue();
		Integer i = 32; // auto boxing 参照型
		int n = i; // auto unboxing
		System.out.println();
	}
}

generics 型の汎用化

class MyData<T> {
	public void getThree(T x){
		System.out.println(x);
		System.out.println(x);
		System.out.println(x);
	}
}

public class MyApp {

	public static void main(String[] args){
		MyData<Integer> i = new MyData<>(); //参照型
		i.getThree(32);

		MyData<String> s = new MyData<>(); //参照型
		s.getThree("What's up");
	}
}

Thread

class MyRunnable implements Runnable {
	@Override
	public void run(){
		for (int i = 0; i < 500; i++){
			System.out.print('*');
		}
	}
}

public class MyApp {

	public static void main(String[] args){
		MyRunnable r = new MyRunnable();

		Thread t = new Thread(r);
		t.start();

		for (int i = 0; i < 500; i++){
			System.out.print('.');
		}
	}
}

無名クラス

public class MyApp {

	public static void main(String[] args){
		new Thread(new Runnable() {
			@Override
			public void run(){
				for (int i = 0; i < 500; i++){
					System.out.print('*');
				}
			}
		}).start();

		for (int i = 0; i < 500; i++){
			System.out.print('.');
		}
	}
}

ラムダ式

new Thread(()-> {
			for (int i = 0; i < 500; i++){
				System.out.print('*');
			}
		}).start();

		for (int i = 0; i < 500; i++){
			System.out.print('.');
		}

String

public class MyApp {

	public static void main(String[] args){
		String s = "abcdef";
		System.out.println(s.length());
		System.out.println(s.substring(2, 5));
		System.out.println(s.replaceAll("ab", "AB"));

		String s1 = "ab";
		String s2 = "ab";

		if(s1.equals(s2)){
			System.out.println("same!");
		}

		if(s1 == s2){
			System.out.println("same!same!");
		}

		String ss1 = new String("abc");
		String ss2 = new String("abc");

		if(ss1 == ss2){
			System.out.println("same!same!same!");
		}
	}
}

printf

public class MyApp {

	public static void main(String[] args){

		int score = 50;
		double height = 165.8;
		String name ="hpscript";

		System.out.printf("name: %-10s, score: %10d, height: %5.2f\n", name, score, height);

		String s = String.format("name: %10s, score: %-10d, height: %5.2f\n", name, score, height);
		System.out.println(s);
	}
}

random

import java.util.Random;

public class MyApp {

	public static void main(String[] args){

		double d = 53.234;
		
		System.out.println(Math.ceil(d));
		System.out.println(Math.floor(d));
		System.out.println(Math.round(d));
		System.out.println(Math.PI);

		Random r = new Random();
		System.out.println(r.nextDouble()); // 0 - 1
		System.out.println(r.nextInt(100));
		System.out.println(r.nextBoolean());
	}
}