JVM
JVM is an acronym for Java Virtual Machine, it is an abstract machine which provides the runtime environment in which java bytecode can be executed. It is a specification.
JVMs are available for many hardware and software platforms (so JVM is platform dependent).
JRE
JRE stands for Java Runtime Environment. It is the implementation of JVM.
JDK
JDK is an acronym for Java Development Kit. It physically exists. It contains JRE + development tools.
public synchronized StringBuffer append(String s): is used to append the specified string with this string. The append() method is overloaded like append(char), append(boolean), append(int), append(float), append(double) etc.
public synchronized StringBuffer insert(int offset, String s): is used to insert the specified string with this string at the specified position. The insert() method is overloaded like insert(int, char), insert(int, boolean), insert(int, int), insert(int, float), insert(int, double) etc.
public synchronized StringBuffer replace(int startIndex, int endIndex, String str): is used to replace the string from specified startIndex and endIndex.
public synchronized StringBuffer delete(int startIndex, int endIndex):is used to delete the string from specified startIndex and endIndex.
public synchronized StringBuffer reverse(): is used to reverse the string.
public int capacity(): is used to return the current capacity.
public void ensureCapacity(int minimumCapacity): is used to ensure the capacity at least equal to the given minimum.
public char charAt(int index): is used to return the character at the specified position.
public int length(): is used to return the length of the string i.e. total number of characters.
public String substring(int beginIndex): is used to return the substring from the specified beginIndex.
public String substring(int beginIndex, int endIndex): is used to return the substring from the specified beginIndex and endIndex.
The java.lang.String class provides a lot of methods to work on string. By the help of these methods, we can perform operations on string such as trimming, concatenating, converting, comparing, replacing strings etc.
Java String is a powerful concept because everything is treated as a string if you submit any form in window based, web based or mobile application.
Let's see the important methods of String class.
Java String toUpperCase() and toLowerCase() method
The java string toUpperCase() method converts this string into uppercase letter and string toLowerCase() method into lowercase letter.
String s="Sachin"; System.out.println(s.toUpperCase());//SACHIN System.out.println(s.toLowerCase());//sachin System.out.println(s);//Sachin(no change in original)
Output
SACHIN sachin Sachin
Java String trim() method
The string trim() method eliminates white spaces before and after string.
String s=" Sachin "; System.out.println(s);// Sachin System.out.println(s.trim());//Sachin
Output
Sachin Sachin
Java String startsWith() and endsWith() method
String s="Sachin"; System.out.println(s.startsWith("Sa"));//true System.out.println(s.endsWith("n"));//true
Output
true true
Java String charAt() method
The string charAt() method returns a character at specified index.
String s="Sachin"; System.out.println(s.charAt(0));//S System.out.println(s.charAt(3));//h
Output
S h
Java String length() method
The string length() method returns length of the string.
String s="Sachin"; System.out.println(s.length());//6
Output
6
Java String intern() method
A pool of strings, initially empty, is maintained privately by the class String.
When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.
String s=new String("Sachin"); String s2=s.intern(); System.out.println(s2);//Sachin
Output
Sachin
Java String valueOf() method
The string valueOf() method coverts given type such as int, long, float, double, boolean, char and char array into string.
int a=10; String s=String.valueOf(a); System.out.println(s+10);
Output:
1010
Java String replace() method
The string replace() method replaces all occurrence of first sequence of character with second sequence of character.
String s1="Java is a programming language. Java is a platform. Java is an Island."; String replaceString=s1.replace("Java","Kava");//replaces all occurrences of "Java" to "Kava" System.out.println(replaceString);
Output:
Kava is a programming language. Kava is a platform. Kava is an Island
Yes, classes like Character, Byte, Integer, Float, Double, Long... called Wrapper classes are immutable. Classes like BigInteger, BigDecimal are also immutable.
String class objects are immutable and hence their contents cannot modified whereas StringBuffer class objects are mutable, so they can be modified.
Moreover the methods that directly manipulate data of the objects are not available in String class and such methods are available in StringBuffer class.
1) String s = "Hello":
2) String s = new String (" Hello");
In the first statement, assignment operator is used to assign the string literal to the String variable s. In this case, JVM first of all checks whether the same object is already available in the string constant pool. If it is available, then it creates another reference to it. If the same object is not available then it creates another object with the content "Hello" and stores it into the string constant pool.
In the second statement, new operator is used create the string object. In this case, JVM always creates a new object without looking in the string constant pool.
Object reference is a unique hexadecimal number representing the memory address of the object. It is useful to access the members of the object.
