JAVA NOTEBOOK (week 2)

KEY POINTS:

1. packages 

2.import

3.API:https://docs.oracle.com/javase/1.5.0/docs/api/    //where to find the usage of packages and methods

1) about import

If you use only a few classes from a package, it is good style to import them individually. This makes it easier to see which classes your program actually uses.
import java.util.Vector;

If you use a lot of classes from a package, it probably makes more sense to use a wild card to import them all.

import java.util.*;

2) 

2.1The  java.lang.String  Class

–String(). Constructs a new String object that represents an empty character sequence.

examples:

–String(char[] value). Constructs a new String object that represents the sequence ofcharacters contained in the character array.

examples:

–String(String original). Constructs a new String object that represents the same sequence ofcharacters as the argument.

examples:

–int length(). Obtains the number of characters in the String.

examples: str.length() 

–char charAt(intindex). Returns the character at the specified index.

examples: str.charAt(idx_end)

–boolean equals(ObjectanObject).Returns true if the specified Object represents a String with the same sequenceof characters.

–int  indexOf(int ch).Returns the index of the first occurrence of the character.

–int  indexOf(String str).Returns the index of the first occurrence of the String.

–boolean  startsWith(String prefix). Checks if the String has the specified prefix.

–String  substring(int beginIndex,int endIndex).Returns a substring. 

–boolean equals() 

examples: x.equals(y)  //x和y引用的是同一类对象且属性内容相等时，返回True

–boolean equalsIgnoreCase()

examples: x.equalsIgnoreCase(y)

-substring

examples:data.substring(idx_start, idx_end)

-indexof

examples: data.indexOf("_", idx_start)

2.2 The  java.util.StringTokenizer  Class 

–StringTokenizer(String str).Constructs a string tokenizer.The tokenizeruses the default delimiter set, white space.

–StringTokenizer(String str,String delim).Constructs a string tokenizer.The argument delimcontains the character delimiters for separating tokens.

examples: StringTokenizer s1= new StringTokenizer(str,"_")

–boolean hasMoreTokens().Tests if there are more tokens to extract.

examples: 

–String nextToken(Stringdelim).Returns the next token in the string.

examples: str.nextToken() 

–int countTokens().Obtains the number of tokens left to be extracted, not the number of tokens inthe string. 

int num = str.countTokens()

3) Practice

3.1

import java.util.*;

public class Lab2_1 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		String data = new String("Mini Discs 74 Minutes (10-pack)_5_9.0");
		String name = null;
		String quantity = null;
		String price = null;
		int numOfTokens = 3;
		int k = 0;
		int idx_start = 0;
		int idx_end = 1;
		while(idx_end < data.length()) {
			if(data.charAt(idx_end)=='_' || idx_end==(data.length()-1)) {
				switch (k) { 
				case 0:
					name = data.substring(idx_start, idx_end);
					k++;
					break;
				case 1:
					quantity = data.substring(idx_start, idx_end);
					k++;
					break;
				case 2:
					price = data.substring(idx_start, idx_end+1);
					k++;
					break;
				}
				
				idx_start = idx_end + 1;
			}
			
			idx_end++;
		}
		
		System.out.println(name);
		System.out.println(quantity);
		System.out.println(price);

3.2

package examples;

import java.util.*;

public class Lab1 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		
		StringTokenizer st = new StringTokenizer("This string has five tokens.");
		int num = st.countTokens();		
		
		while(st.hasMoreTokens()) {
			System.out.print(st.nextToken() + ", ");
		}
		System.out.println();
		
		for(int i=0;i<num;i++) {
			System.out.print(st.nextToken() + ", ");
		}
        	System.out.println();
		
		// TODO Auto-generated method stub
		String s1 = new String("hello");
		String s2 = new String("Hello"); 
		String s3 = new String("hello");
		
		System.out.println(s1 + s1.length() + s2 + s2.length() + s3 + s3.length());		
                System.out.println(1+2+3);
		System.out.println(""+1+2+3);
		
		System.out.println("s1==s2 : " + (s1==s2));
		System.out.println("s1==s3 : " + (s1==s3));
		System.out.println("s2==s3 : " + (s2==s3));	
		
		System.out.println("s1.equals(s2) : " + s1.equals(s2));
		System.out.println("s1.equals(s3) : " + s1.equals(s3));
		System.out.println("s2.equals(s3) : " + s2.equals(s3));
		System.out.println("s1.equalsIgnoreCase(s2) : " + s1.equalsIgnoreCase(s2));
		System.out.println("s1.equalsIgnoreCase(s3) : " + s1.equalsIgnoreCase(s3));
		System.out.println("s2.equalsIgnoreCase(s3) : " + s2.equalsIgnoreCase(s3));
	}

}

3.3

package examples;

public class Lab2_2 {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String data = new String("Mini Discs 74 Minutes (10-pack)_5_9.0");
		int idx_start = 0;
		int idx_end = 0;
		
		idx_end = data.indexOf("_", idx_start);
		String name = data.substring(idx_start, idx_end);
		idx_start = idx_end + 1;
		idx_end = data.indexOf("_", idx_start);
		String quantity = data.substring(idx_start, idx_end);
		idx_start = idx_end + 1;
		idx_end = data.indexOf("_", idx_start);
		String price = data.substring(idx_start, data.length());
		
		System.out.println(name);
		System.out.println(quantity);
		System.out.println(price);
		
	}

}