Working with String Constructors in Java

In this lesson, you will learn

• String Constructors
• String Concatenation
• toString( ) Method
• Methods of String Class

String Constructors

In Java, strings are objects that represent sequences of characters. The String class provides several constructors for creating strings.

Here are the primary constructors used to create strings in Java, along with examples.

1. Creating String from Default Constructor

Creates an empty string. This constructor is not often used since strings are immutable, and you can’t change an empty string afterward.

String str = new String();
// Creates an empty string

2. Creating String from an Array of Characters

To create a String initialized by an array of characters, use the following constructor

Constructor: public String(char chars[ ])

Example

char[] charArray = {'H', 'e', 'l', 'l', 'o'};
String str = new String(charArray);
/* Creates a string "Hello" from an 
array of characters */

You can specify a subrange of a character array as an initializer using the following constructor:

Constructor: public String(char chars[ ], int startIndex, int numChars)

Here, startIndex specifies the index at which the subrange begins, and numChars specifies the number of characters to use.

Example

char chars[] = { 'a', 'b', 'c', 'd', 'e', 'f' };
String s = new String(chars, 2, 3);
//This initializes s with the characters cde.

3. Creating String from String Objects

You can construct a String object that contains the same character sequence as another String object using this constructor.

Constructor: public String(String strObj)

Here, strObj is a String object.

Example

package ch11.l2;

public class CreateString {
	public static void main(String[] args) {
		char c[] = {'J', 'a', 'v', 'a'}; 
		String s1 = new String(c);
		String s2 = new String(s1);
		System.out.println(s1);
		System.out.println(s2);
		System.out.println("s1==s2: "+(s1==s2));
		//check equality of objects
	}
}

Output

Java
Java
s1==s2: false

4. Creating String from Byte Array

Constructs a new String by decoding an array of bytes using the platform’s default charset.

Constructors:

public String(byte chrs[ ])
public String(byte chrs[ ], int startIndex, int numChars)

Here, chrs specifies the array of bytes. The second form allows you to specify a subrange.

Example

package ch11.l2;

public class MakeStringFromByteArray {

	public static void main(String[] args) {
		byte ascii[] = {65, 66, 67, 68, 69, 70 };
		String s1 = new String(ascii);
		System.out.println(s1);
		String s2 = new String(ascii, 2, 3);
		System.out.println(s2);
	}
}

Output

ABCDEF
CDE

5. Creating String from a Substring

• Constructs a new String that contains characters from a substring(int beginIndex) and substring(int beginIndex, int endIndex) of an existing String.
• The substring begins at the specified beginIndex and extends to the character at index endIndex - 1.

Example

String original = "Hello, World!";
String str = new String(original.substring(0, 5));
/* Creates a string "Hello" from a 
substring of "Hello, World!" */

6. StringBuilder or StringBuffer

• Constructs a new String that represents the same sequence of characters as the content of the StringBuilder or StringBuffer.

• You can construct a String by using the constructor shown here

Constructors

public String(StringBuffer strBufObj)

public String(StringBuilder strBuildObj)

Example

StringBuilder sb = new StringBuilder("Hello");
String str = new String(sb);
// Creates a string "Hello" from a StringBuilder

String Concatenation

• The ‘+’ operator, which concatenates two strings, produces a String object as the result

Example

String myage = "21";
String s1 = "He is " + myage + " years old.";
System.out.println(s1); //"He is 21 years old."

String Concatenation with Other Data Types

int myage = 21;
String s1 = "He is " + myage + " years old.";
System.out.println(s1); //"He is 21 years old."

String s = "four: " + 2 + 2;
System.out.println(s); //four: 22
String s = "four: " + (2 + 2); //four: 4

toString( ) Method

• In Java, the toString() method is a public method provided by the Object class, which is the superclass of all classes in Java.
• Its primary purpose is to return a string representation of an object.
• When you create a class in Java, it inherits the toString() method from the Object class.
• This method can be overridden in any custom class to provide a meaningful string representation of an object, which can be particularly helpful for debugging or logging purposes.

Example 1: Without Using ‘toString()’ Method

package ch11.l2;

class Person{
	public String name;
	public int age;
	public String gender;
	
	public Person(String name, int age, String gender) {
		this.name = name;
		this.age = age;
		this.gender = gender;
	}	
}

public class ToStringExample {

	public static void main(String[] args) {
		Person p1=new Person("Amit", 27, "Male");
		Person p2=new Person("Riya", 23, "Female");
		System.out.println(p1);//compiler writes here p1.toString()
		System.out.println(p2);//compiler writes here p1.toString()
	}
}

Output

ch11.l2.Person@7cc355be
ch11.l2.Person@6e8cf4c6

Example 2: Adding ‘toString()’ Method in a Class

package ch11.l2;

class Person{
	public String name;
	public int age;
	public String gender;
	
	public Person(String name, int age, String gender) {
		this.name = name;
		this.age = age;
		this.gender = gender;
	}

	@Override
	public String toString() {
		return "Person [name=" + name + ", age=" + age + ", "
				+ "gender=" + gender + "]";
	}	
	
}

public class ToStringExample {

	public static void main(String[] args) {
		Person p1=new Person("Amit", 27, "Male");
		Person p2=new Person("Riya", 23, "Female");
		System.out.println(p1);//compiler writes here p1.toString()
		System.out.println(p2);//compiler writes here p1.toString()
	}
}

Output

Person [name=Amit, age=27, gender=Male]
Person [name=Riya, age=23, gender=Female]

Methods of String Class

Below is a summarized table of some commonly used String methods, categorized by their purpose with examples.

End of the lesson….enjoy learning

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