數組實例：素數

import java.util.Scanner;

public class Prime {

	public static void main(String[] args) {
		/* 	求素數等一系列問題 */
		// 1.從2到x-1測試是否可以整除
		Scanner in = new Scanner(System.in);
		int x = in.nextInt();
		boolean isPrime = true;
		if(x == 1)
			isPrime = false;
		for(int i = 2; i < x ; i++) {
			if(x % i == 0) {
				isPrime = false;
				break;
			}
		}
		if(isPrime)
			System.out.println(x+" is prime.");
		else
			System.out.println(x+" is not prime.");

	}
}

import java.util.Scanner;

public class Prime {

	public static void main(String[] args) {
		/* 	求素數等一系列問題 */
		// 2.去掉偶數後，從3到x-1，每次+2.
		Scanner in = new Scanner(System.in);
		int x = in.nextInt();
		boolean isPrime = true;
		if(x == 1 || x % 2 == 0 && x != 2)
			isPrime = false;
		else {
			for(int i = 3; i < x ; i+=2) {
				if(x % i == 0) {
					isPrime = false;
					break;
				}
			}
		}
		if(isPrime)
			System.out.println(x+" is prime.");
		else
			System.out.println(x+" is not prime.");
	}
}

import java.util.Scanner;

public class Prime {

	public static void main(String[] args) {
		/* 	求素數等一系列問題 */
		// 3.去掉偶數後，從3到sqrt(x)，每次+2.
		Scanner in = new Scanner(System.in);
		int x = in.nextInt();
		boolean isPrime = true;
		if(x == 1 || x % 2 == 0 && x != 2)
			isPrime = false;
		else {
			for(int i = 3; i < Math.sqrt(x) ; i+=2) {
				if(x % i == 0) {
					isPrime = false;
					break;
				}
			}
		}
		if(isPrime)
			System.out.println(x+" is prime.");
		else
			System.out.println(x+" is not prime.");
	}
}

import java.util.Scanner;

public class Prime {

	public static void main(String[] args) {
		/* 	求素數等一系列問題 */
		// 4.判斷是否能被已知的且<x的素數整除，構造前50個素數的表。
		Scanner in = new Scanner(System.in);
		int[] primes = new int[50];
		primes[0] = 2;
		int cnt = 1;
		MAIN_LOOP:
			for(int x = 3; cnt < primes.length; x++) {
				for(int i = 0; i < cnt; i++) {
					if(x % primes[i] == 0) {
						continue MAIN_LOOP;
					}
				}
				primes[cnt++] = x;
			}
		for(int k : primes) {
			System.out.print(k+" ");
		}
	}
}

import java.util.Scanner;

public class Prime {

	public static void main(String[] args) {
		/* 	求素數等一系列問題 */
		// 5.構造素數表：令x爲2，將2x、3x、4x直至ax<n的數標記爲非素數，令x爲下一個沒有被標記爲非素數的數。
		Scanner in = new Scanner(System.in);
		boolean[] isPrime = new boolean[100];
		for(int i = 2; i < isPrime.length; i++) {
			isPrime[i] = true;
		}
		
		for(int i = 2; i < isPrime.length; i++) {
			if(isPrime[i]){
				for(int k = 2; i * k < isPrime.length; k++) {
					isPrime[i*k] = false;
				}
			}
		}
		for(int i = 0; i < isPrime.length; i++) {
			if(isPrime[i])
				System.out.print(i+" ");
		}
	}
}