Strings in Java

What is a String

A String holds text — a sequence of characters like "Hello". In Java, String is a class (written with a capital S), so it is a reference type with many useful built-in methods. Strings are one of the most-used types in any program.

Creating strings

String a = "Hello";               // literal (preferred)
String b = new String("World");   // with new (rarely needed)
System.out.println(a + " " + b);

The usual way is a string literal in double quotes. Using new String(...) also works but is rarely needed and makes an extra object.

Common string methods

String s = "CodingEasily";
System.out.println(s.length());         // 12
System.out.println(s.toUpperCase());    // CODINGEASILY
System.out.println(s.substring(0, 6));  // Coding
System.out.println(s.charAt(0));        // C
System.out.println(s.indexOf("Easy"));  // 6

length() gives the count, substring() extracts part, charAt() gets one character, and indexOf() finds a position. There are many more.

Strings are immutable

String s = "hello";
s.toUpperCase();          // does NOT change s
System.out.println(s);    // still hello

s = s.toUpperCase();      // reassign to keep the result
System.out.println(s);    // HELLO

A String cannot be changed after creation. Methods like toUpperCase() return a new String, leaving the original untouched. You must reassign to keep the result.

Comparing strings

String a = "hi";
String b = "hi";
String c = new String("hi");

System.out.println(a.equals(c));   // true  (same text)
System.out.println(a == c);        // false (different objects)

Use equals() to compare text content. == compares references (whether they are the same object), which is usually not what you want for strings.

Joining strings

String first = "Coding";
String full = first + "Easily";     // with +
String also = first.concat("Easily"); // with concat
System.out.println(full);

The + operator and concat() both join strings. For building a long string inside a loop, prefer a StringBuilder for better performance.

Common mistakes

• Using == to compare text. Use equals() for content comparison.
• Expecting a method to change the string. Strings are immutable — reassign the result.
• Using length without brackets. Strings use length() (a method), arrays use length.
• Out-of-range substring. Indexes must be within the string, or you get an error.

Summary

• A String is a class holding text, with many built-in methods.
• Create with a literal in double quotes; new String is rarely needed.
• Strings are immutable — methods return a new String.
• Compare content with equals(), not ==.
• Join with + or concat(); use StringBuilder for heavy building.

Next, learn efficient editing with String vs StringBuffer vs StringBuilder, or reverse text in reverse a string.