reference:http://examples.javacodegeeks.com/core-java/lang/string/java-string-class-example/

1. Introduction

In this example we are going to discuss about the basic characteristics of Java String ClassString is probably one of the most used types in Java programs. That’s why Java provides a number of API methods that make String manipulation easy and efficient, straight out of the box. Strings are so important that even in the latest Java releases (including 7 and 8), several changes have been made to its class methods and its internal representation, improving it even further in terms of performance and security.

2. String Class basic methods

String is simply a sequence of characters. As a matter of fact, a String Object is backed by a char array. Consequently, it is not null terminated, like in C/C++.

Here is how you can create a String

1 String str= "Hello World";

"Hello World" is called a String literal. In a Java program, everything between two double quotes is a String literal. Literals are implemented as instances of String class. As you can see, you can conveniently initialize a String Object like a primitive type, e.g int i = 0;.

There is no need to do:

1 String str = new String("Hello World");

There is a difference between these two initialization methods, although the result is the same : A String with value “Hello World”. But more on that in just a bit.

For now, here is a simple main with the most important String API methods:

StringClassExample.java

001 package com.javacodegeeks.core.lang.string;
002  
003 public class StringClassExample {
004  
005     public static void main(String[]args){
006         //Initialization with literal
007         String str1 = "Hello World";
008         System.out.println("str1:"+str1);
009  
010         //Initialization with char array
011         char arr[] = {'H','e','l','l','o'};
012         String str2 = new String(arr);
013         System.out.println("str2:"+str2);
014  
015         //Concatenation using + operator
016         String str3 = "World";
017         str3 = str2 + " " + str3;
018         System.out.println("str3:"+str3);
019  
020         //find out the length of a string
021         System.out.println(str3.length());
022  
023         //You can even apply that to literals, as with all String API methods
024         //As we said. literals are implemented as String instances
025         System.out.println("Length: "+"abcdefg".length());
026  
027         //Substring from position 2 to position 10
028         String c = str1.substring(2,10);
029         System.out.println("Substring :"+c);
030  
031         //Substring from position 1 to position 4
032         System.out.println("Literal Substring :"+"abcdefghigklm".substring(1,4));
033  
034         // Get the charcter array of the string.
035         char[] chars = c.toCharArray();
036         System.out.println("Char array : ["+chars[0]+","+chars[1]+","+chars[2]+"]");
037  
038         //find the first index of a char inside a string
039         int i = str1.indexOf('W');
040         System.out.println("Index of 'W':"+i);
041  
042         //find the first index of a string inside another string after a certain position
043         i = str1.indexOf("orld",5);
044         System.out.println("Index of 'orld':"+i);
045  
046         //find the last index of a string inside another string
047         i = str1.lastIndexOf("l");
048         System.out.println("LAST Index of 'l':"+i);
049  
050         //find the last index of a string inside another string after a certain position
051         // - like scanning the string backwards
052         i = str1.lastIndexOf("l",7);
053         System.out.println("LAST Index of 'l':"+i);
054  
055         //find a character in a certain position
056         char cr = str1.charAt(5);
057         System.out.println("Character at position 5:"+cr);
058  
059         //Lower case
060         System.out.println("ABCEFAFA".toLowerCase());
061  
062         //Upper case
063         System.out.println("abcasipasc".toUpperCase());
064  
065         //replace occurrences of a character
066         str1 = str1.replace('o','0');
067         System.out.println(str1);
068  
069         //Trim white spaces from the end and the beginning
070         String str4 = "    Java";
071         System.out.println(str4);
072         System.out.println(str4.trim());
073  
074         //Split !!!
075         String str5= "Java is great";
076         String[] strArray = str5.split(" ");
077  
078         System.out.println(strArray[0]+","+strArray[1]+","+strArray[2]);
079  
080         str5= "Java-is-great";
081         strArray = str5.split("-");
082         System.out.println(strArray[0]+","+strArray[1]+","+strArray[2]);
083  
084         str5= "Java is great";
085         strArray = str5.split("/*");
086         System.out.println(strArray[0]+","+strArray[1]+","+strArray[2]+","+strArray[3]+","+strArray[4]+
087                 ","+strArray[5]+","+strArray[6]+","+strArray[7]+","+strArray[8]);
088  
089         //contains and equals
090         System.out.println("Contains :" "qwerty".contains("ert"));
091         System.out.println ("Equals :"+str5.equals("java is great"));
092         System.out.println ("Equals ignore case:"+str5.equalsIgnoreCase("java is great"));
093  
094         // Compare lexicographically two strings
095         System.out.println ("Compare:"+str5.compareTo("abc"));
096  
097         //comparison attempts
098         String s1 = "abc";
099         String s3 = new String("abc");
100  
101         System.out.println(s1==s3);
102         System.out.println(s1.equalsIgnoreCase(s3));
103  
104     }
105  
106 }

This is the output of the above program:

str1:Hello World
str2:Hello
str3:Hello World
11
Length: 7
Substring :llo Worl
Literal Substring :bcd
Char array : [l,l,o]
Index of 'W':6
Index of 'orld':7
LAST Index of 'l':9
LAST Index of 'l':3
Character at position 5:
abcefafa
ABCASIPASC
Hell0 W0rld
Java
Java
Java,is,great
Java,is,great
,J,a,v,a, ,i,s,
Contains :true
Equals :false
Equals ignore case:true
Compare:-23
false
true

From the above program is clear that Java designers decided to treat Strings somewhat differently from other Objects. For example you can initialize them like a primitive, e.g String a="abc" and you can concatenate two strings using + operator, like you would add twoints (looks like overloading + operator in C++).

The comparison attempts section of the code might seem a little fuzzy, but it will get clear in the next section. What you should take away from it now, is that you SHOULD NEVER try to compare the contents of Strings using == operator. You are only comparing reference equality, not content equality. You MUST use equals or equalsIgnoreCase.

3. Other characteristics of String objects

String objects are immutable. This means that once a String is created, its contents cannot be changed. In the above example, every time we attempt to change its contents, e.g when concatenating, a new String object is created representing the result. Additionally, String class is final, so you cannot override its behavior.

Immutability was mostly chosen for security reasons and for performance. It also means that two different thread can share the same String and manipulate it as they want, not having to synchronize anything, because every time they make a change in the original string, a new one is created, while the old one remains untouched.

Now let’s see this :

1 String s1 = "abc";
2 String s2= "abc";
3  
4 String s3 = new String("abc");
5  
6 System.out.println(s1==s2);
7 System.out.println(s1==s3);

This outputs:

true
false

Literals are stored in a special place in memory, called a String pool, of course in the form of String Objects. In that pool, a Stringobject with value “abc” is only created and stored once. Any other String that gets the value “abc” (statically – hard coded) will reference the same String object. So, every time you create a String using a literal, the system will search that pool and checks if the value of the literal exists in an object of the pool. If it does, it sends back the reference to that matching object, if not it creates a new Object and stores it in the pool. So, String references, initialized with the same literals, will point to the same String object. This technique was used to save precious memory, as it shares as much common data as possible.

Now, you can also see another reason why Strings are immutable. Imagine thread A creating a local string “abc” and then a second thread B creating his own local string “abc”. These two threads will share the same String object… If String was mutable, then if A changed the string, the change would affect thread B, but in a meaningless (put catastrophic) way.

When creating a String using new, you explicitly create a brand new object in the heap. This is also the case for non hard codedString initialization, for example, if you are reading input Strings from a source. These String Objects will not be stored in the pool. Imagine that you create an application that has to hold addresses for users living in Greece. There are four million people living in Athens, so consider the massive waste of space should you store four million String objects with value “Athens”. In order to pool those non hard coded Strings, there is an API method called intern, and can be used like so:

01 String s1 = "abc";
02 String s2= "abc";
03  
04 String s3 = new String("abc");
05  
06 System.out.println(s1==s2);
07 System.out.println(s1==s3);
08  
09 s3 = s3.intern();
10 System.out.println(s1==s3);

This will now output:

true
false
true

When calling intern, the system follows the same procedure as if we did s3 = "abc", but without using literals.

But be careful. Before Java 7, this pool was located in a special place in the Java Heap, called PermGen. PermGen is of fixed size, and can only hold a limited amount of string literals. So, interning should be used with ease. From Java 7 onward, the pool will be stored in the normal heap, like any other object (making them eligible for garbage collection), in a form of a hashmap and you can adjust its size using -XX:StringTableSize option. You could create your own String pool for that matter, but don’t bother.

This is only one of the aspects that Java creators changed in the String class. Even more radical changes ware made, including the internal String representation (it now has two less static fields).

Download the Eclipse Project

This was an example of Java String Class. You can download the Eclipse Project of this example here : StringClassExample.zip

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