Java語言復習

使用 Eclipse 寫 Java 程式

Step1

Step2

Step3

Step4

Java語言的Hello world程式

public class Ex1Class {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("Hello world");
    }
}

說明

Java程式的組成

寫程式基本上就是由輸入資料經過運算然後得到輸出。這樣的過程可以簡化成五個部份：一、宣告變數。二、進行運算。三、控制流程。四、判斷資料的值。五、輸入及輸出。

import java.util.Scanner;
public class Ex2Class {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.printf("Input ID:");
        int nID = sc.nextInt();
        int even = 0, odd = 0, digits=0;
        int num=nID;
        while (num >0){
            digits++;
            if (num%2==0)
                even++;
            num /=10;
        }
        odd = digits - even; 
        System.out.printf("My ID: %9d -- %d digits %d odd nums %d even nums\n", nID, digits, odd, even);
    }
}

說明

Java Loop

String [] a = {"1","3","5","7","9","11"}; 
//5.0前的for迴圈寫法 
for(int i = 0 ; i < a.length ; i++){ 
    String temp = a[i]; 
    System.out.print(temp + ", "); 
} 
//5.0後for迴圈也可以用這種新的寫法（for-each) 
for(String temp : a){ 
   System.out.print(temp + ", "); 
}

表示式及指定式

從命令列輸入值

double d1, d2;
int a1, a2;

d1 = Double.parseDouble(args[0]);
d2 = Double.parseDouble(args[1]);
a1 = Integer.parseInt(args[0]);
a2 = Integer.parseInt(args[1]);

從檔案輸入

try
{
    //In
    FileReader reader = new FileReader("math.txt");
    BufferedReader sr = new BufferedReader(reader);

    String line = sr.readLine();
    while (line != null && line.length() > 0)
    {
        int score = Integer.parseInt(line);
        line = sr.readLine();
    }
    sr.close();        

}
catch(FileNotFoundException fe)
{
    System.out.printf(fe.getMessage());
}
catch(IOException ie)
{
    System.out.printf(ie.getMessage());
}

從檔案輸出

try
{
    //Out
    FileWriter writer = new FileWriter(result.txt);
    PrintWriter pw = new PrintWriter(writer);
        System.out.printf("%2d => %5.2f\n", score, modified);
                pw.printf("%5.2f", modified);pw.println();    
    pw.close();
}
catch(FileNotFoundException fe)
{
    System.out.printf(fe.getMessage());
}
catch(IOException ie)
{
    System.out.printf(ie.getMessage());
}

產生亂數

Random random = new Random();
int num= (random.nextInt()& Integer.MAX_VALUE)%1000+1001;

把字串切開

String[] numStr = line.split(" ");

Java.util.ArrayList

import java.util.ArrayList;

public class ArrayListDemo {
   public static void main(String[] args) {

   // create an empty array list with an initial capacity
   ArrayList<Integer> arrlist = new ArrayList<Integer>(5);

   // use add() method to add elements in the deque
   arrlist.add(20);
   arrlist.add(15);
   arrlist.add(30);
   arrlist.add(45);

   System.out.println("Size of list: " + arrlist.size());

   // let us print all the elements available in list
   for (Integer number : arrlist) {
   System.out.println("Number = " + number);
   }  

   // Removes element at 3rd position
   arrlist.remove(2);

   System.out.println("Now, Size of list: " + arrlist.size());

   // let us print all the elements available in list
   for (Integer number : arrlist) {
   System.out.println("Number = " + number);
   }
   }
}

Java.util.LinkedList

import java.util.*;

public class LinkedListDemo {

   public static void main(String[] args) {

   // create a LinkedList
   LinkedList list = new LinkedList();

   // add some elements
   list.add("Hello");
   list.add(2);
   list.add("Chocolate");
   list.add("10");

   // print the list
   System.out.println("LinkedList:" + list);

   // remove the element at index 2
   System.out.println("Element to be removed:" + list.remove(2));

   // print the list
   System.out.println("LinkedList:" + list);
   }
}