String Constructor in Java

String Tutorial

➤ Immutable Class in Java
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➤ Why String is Immutable in Java

String Constructor in Java. The string class has a total of 15 constructors among them below 8 are regularly used constructors. All these constructors are overloaded constructors with siblings.

  1. public String()
  2. public String(String s)
  3. public String(StringBuffer sb)
  4. public String(StringBuilder sb)
  5. public String(char[] ch)
  6. public String(char[] ch, int offset, int count)
  7. public String(byte[] b)
  8. public String(byte[] b, int offset, int length)

Other constructors of String class are:-

  1. public String(int[] codePoints, int offset, int count)
  2. public String(byte bytes[], Charset charset)
  3. public String(byte bytes[], String charsetName) throws UnsupportedEncodingException
  4. public String(byte bytes[], int offset, int length, Charset charset)
  5. public String(byte bytes[], int offset, int length, String charsetName) throws UnsupportedEncodingException

Two more string constructor in Java are available, but they are deprecated since the 1.1 version. They are:-

  1. public String(byte ascii[], int hibyte)
  2. public String(byte ascii[], int hibyte, int offset, int count)

Important String Constructor in Java

Now let us important string constructor in Java with examples, and then we will see rules to be followed while using string constructor.

String()

It creates an empty string object with the content "" i.e. the string having 0 length or empty character sequence. This constructor is unnecessary since Strings are immutable.

public String()

public class Test1 {
   public static void main(String[] args) {
   String s1 = new String();
   System.out.println("String value: "+ s1);
   }
}

Output:-

String value:

Note that it is not equal to the null string referenced variable.

String s = ""; // empty string
String s = null; // null reference
// Both are different

In the first line (String s = ""; ) one string object is created with 0 lengths but in the next length String object is not there.

String(String s)

It creates a String object with given String object characters. It performs a String copy operation.

public String(String s)

public class Test2 {
   public static void main(String[] args) {
      String s = "KnowProgram";

      // string copy
      String s1 = new String(s);

      // creating string directly using string literal
      String s2 = new String("Learn Java");

      // reference assignment
      String s3 = s2;

      System.out.println("Address of String s: " + 
                          System.identityHashCode(s));
      System.out.println("String s value: " + s);

      System.out.println("Address of String s1: " + 
                          System.identityHashCode(s1));
      System.out.println("String s1 value: " + s1);

      System.out.println("Address of String s2: " + 
                          System.identityHashCode(s2));
      System.out.println("String s2 value: " + s2);

      System.out.println("Address of String s3: " + 
                          System.identityHashCode(s3));
      System.out.println("String s3 value: " + s3);

      System.out.println(s == s1);
      System.out.println(s2 == s3);
   }
}

Output:-

Address of String s: 1259475182
String s value: KnowProgram
Address of String s1: 1072591677
String s1 value: KnowProgram
Address of String s2: 1523554304
String s2 value: Learn Java
Address of String s3: 1523554304
String s3 value: Learn Java
false
true

// string copy
String s1 = new String(s);
String s2 = new String("Learn Java");

In string copy two different String object is created with same data, it is like a file copy operations in the operating system.

// string reference assignment
String s3 = s2;

In string reference assignment current object reference is copied to the destination variable, a new object is not created.

String(StringBuffer sb)

It creates a new String object with the given StringBuffer object’s data. It performs string to copy from StringBuffer object to String object.

public String(StringBuffer buffer)

public class Test3 {
   public static void main(String[] args) {
      StringBuffer sb = new StringBuffer();
      sb.append("Java String");
      
      String str = new String(sb);
      System.out.println(str);
   }
}

Output:-

Java String

String(StringBuilder sb)

It creates a new String object with the given StringBuilder object content. It performs string copy operation from StringBuilder object to String object.

public String(StringBuilder builder)

public class Test4 {
   public static void main(String[] args) {
      StringBuilder sb = new StringBuilder();
      sb.append("Learn Java");

      String str = new String(sb);
      System.out.println(str);
   }
}

Output:-

Learn Java

String(char[] ch)

It creates a String object with the given char array values. It performs string copy operation from char[] object to a String object.

public String(char value[])

public class Test5 {
   public static void main(String[] args) {
      char[] ch = {'K','n','o','w','p','r','o','g','r','a','m'};
      String s = new String(ch);
      System.out.println("String value: "+ s);
   }
}

Output:-

String value: Knowprogram

String(char[] ch, int offset, int count)

It creates a new object with the given count number of characters from the given offset in the char[] object. Here offset is the starting index from which characters must be copied.

public String(char value[], int offset, int count)

  • offset:- the index from characters should be copied.
  • count:- the number of characters should be copied from the offset.

Note that the index of an array starts from 0.

class Test6 {
   public static void main(String[] args) {
      char[] ch = {'K','n','o','w','P','r','o','g','r','a','m'};
      String s = new String(ch, 4, 7);
      System.out.println("String value: "+ s);
   }
}

Output:-

String value: Program

The value of offset is 4 so, the new string will be copied from the 4th index of ch[]. The element at the 4th index in the ch[] is ‘p’. The value of count is 7, so from ‘p’ 7 elements will be copied. Finally, the new string is “Program”.

String(byte[] b)

It creates a new String object by copying the given byte[] numbers by converting them into their ASCII characters.

public String(byte[] bytes)

class Test7 {
   public static void main(String[] args) {
      byte[] b = {65,66,97,98};
      String s = new String(b);
      System.out.println("String value: "+ s);
   }
}

Output:-

String value: ABab

String(byte[] b, int offset, int count)

It creates a new String object with the given count number of bytes from the given offset in the byte[]. All bytes are stored in their ASCII character form.

public String(byte bytes[], int offset, int length)

class Test8 {
   public static void main(String[] args) {
      byte[] b = {65,66,97,98};
      String s = new String(b, 2, 2);
      System.out.println("String value: "+ s);
   }
}

Output:-

String value: ab

These were the important constructors given in the String class. Now, let us see some rules regarding String object creation through constructors.

Rules for String Constructor in Java with offset and count

Example of String Constructor in Java with offset and count as parameter values,

  • public String(char[] ch, int offset, int count)
  • public String(byte[] b, int offset, int length)

1) The given offset argument value must be within the range of [0 to LengthOfString], else it leads to run time exception:- StringIndexOutOfBoundsException.

The new string object will be copied from the offset. The offset gives the index number, so the index should be >=0 and less than the length of the string.

public class Rule1 {
   public static void main(String[] args) {
      byte[] b = { 65, 66, 97, 98 };
      String s1 = new String(b, -1, 2); // Exception
      String s2 = new String(b, 0, 2); // Valid
      String s3 = new String(b, 5, 1); // Exception

      char[] ch = { 'a', 'b', 'c', 'd' };
      String s4 = new String(ch, -2, 2); // Exception
      String s5 = new String(ch, 0, 2); // Valid
      String s6 = new String(ch, 4, 2); // Exception
   }
}

Here, we get java.lang.StringIndexOutOfBoundsException. Note that the exception is StringIndexOutOfBoundsException not the ArrayIndexOutOfBoundsException.

2) The given count value depends on offset. The new string object copied from the offset to the given number of count characters. Therefore it also should be valid.

public class Rule2 {
   public static void main(String[] args) {
      char[] ch = { 'a', 'b', 'c', 'd' };
      String s = new String(ch, 0, -4); // Exception
      String s1 = new String(ch, 0, 0); // Valid
      String s2 = new String(ch, 0, 4); // Valid
      String s3 = new String(ch, 0, 5); // Exception
      String s4 = new String(ch, 0, 6); // Exception
   }
}

Example with valid offset and count values.

public class Example {
   public static void main(String[] args) {
      char[] ch = { 'a', 'b', 'c', 'd' };
      String s1 = new String(ch, 0, 4);
      System.out.println(s1);
      String s2 = new String(ch, 1, 3);
      System.out.println(s2);
      String s3 = new String(ch, 2, 2);
      System.out.println(s3);
      String s4 = new String(ch, 3, 1);
      System.out.println(s4);
   }
}

Output:-

abcd
bcd
cd
d

Working with null in String Constructor in Java

1) We can’t pass null as argument directly to the constructors of the string class, it leads to compile-time error: ambiguous error because it is matched with all parameters of String constructor in Java.

public class Rule1 {
   public static void main(String[] args) {
      String s = new String(null);
      System.out.println("String value: " + s);
   }
}

Compile-time error: reference to String is ambiguous

error: reference to String is ambiguous
	String s = new String(null);
	           ^
  both constructor String(StringBuffer) in String and
  constructor String(StringBuilder) in String match
1 error

The null is a default value of all referenced variables. So, most of the constructors of the string class can take null as an argument. The compiler can’t decide that this null is passed to which constructor, so it gives an ambiguous error. Read the error message given by the compiler:- both constructor String(StringBuffer) in String and constructor String(StringBuilder) in String match.

2) We can’t create String object with null it leads to runtime error:- java.lang.NullPointerException

In rule1, we have seen that the compiler gives an ambiguous error because the compiler didn’t decide which constructor should be used. But by using casting we can convert the null into string-type null. In this case, the compiler didn’t give any error because this time compiler knows that (string)null is used to call constructor having string parameter.

But in this case, the JVM will throw NullPointerException. To create a new string object JVM need to perform the copy operation. To copy the string data it should point to the referenced variable. Here, due to null, the JVM can’t perform the copy operation.

public class Rule2 {
   public static void main(String[] args) {
      String str = null;
      String s1 = new String(str); // Exception
      String s2 = new String((String) null); // Exception
      String s3 = new String((StringBuffer) null); // Exception
      String s4 = new String((StringBuilder) null); // Exception
   }
}

Exception in thread “main” java.lang.NullPointerException

We can create a null String referenced variable (in the above program, str) but we can’t create a String object with null. It leads to run time error.

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