The differences between String, StringBuffer and StringBuilder

String

String is immutable(once created can not be changed) object. The object created as a String is stored in the Constant String Pool.

Every immutable object in Java is thread safe ,that implies String is also thread safe . String can not be used by two threads simultaneously. String once assigned can not be changed.

StringBuffer

StringBuffer is mutable means one can change the value of the object. The object created through StringBuffer is stored in the heap. StringBuffer has the same methods as the StringBuilder , but each method in StringBuffer is synchronized that is StringBuffer
is thread safe.

Due to this it does not allow two threads to simultaneously access the same method . Each method can be accessed by one thread at a time .

But being thread safe has disadvantages too as the performance of the StringBuffer hits due to thread safe property . Thus StringBuilder is faster than the StringBuffer when calling the same methods of each class.

String Buffer can be converted to the string by using toString() method.

StringBuffer demo1 = new StringBuffer("Hello") ;

// The above object stored in heap and its value can be changed .

demo1=new StringBuffer("Bye");

// Above statement is right as it modifies the value which is allowed in the StringBuffer.

StringBuilder

StringBuilder is same as the StringBuffer , that is it stores the object in heap and it can also be modified . The main difference between the StringBuffer and StringBuilder is that StringBuilder is also not thread safe.

StringBuilder is fast as it is not thread safe .

StringBuilder demo2= new StringBuilder("Hello");

// The above object too is stored in the heap and its value can be modified

demo2=new StringBuilder("Bye");

// Above statement is right as it modifies the value which is allowed in the StringBuilder

public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
/** The value is used for character storage. */
private final char value[];

/** Cache the hash code for the string */
private int hash; // Default to 0

......

}

abstract class AbstractStringBuilder implements Appendable, CharSequence {
/**
* The value is used for character storage.
*/
char[] value;

int count;

AbstractStringBuilder() {
}

AbstractStringBuilder(int capacity) {
value = new char[capacity];
}

......

}

public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{

/**
* A cache of the last value returned by toString. Cleared
* whenever the StringBuffer is modified.
*/
private transient char[] toStringCache;

/** use serialVersionUID from JDK 1.0.2 for interoperability */
static final long serialVersionUID = 3388685877147921107L;

public StringBuffer() {
super(16);
}

public StringBuffer(int capacity) {
super(capacity);
}

public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}

@Override
public synchronized int length() {
return count;
}

@Override
public synchronized int capacity() {
return value.length;
}

......

}

public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{

/** use serialVersionUID for interoperability */
static final long serialVersionUID = 4383685877147921099L;

public StringBuilder() {
super(16);
}

public StringBuilder(int capacity) {
super(capacity);
}

public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}