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Java - String Methods

📌 What You Will Learn
  • Why String objects are immutable
  • String vs StringBuffer vs StringBuilder
  • The most important methods of String
  • == vs equals() — the classic interview question
  • How to split, join and manipulate text

String Methods is a core concept of the Java language. This lesson explains Most Important String Methods, == vs equals() — The Five-Object Version and uick Practice Questions with complete, runnable code examples, clear step-by-step explanations, and common mistakes to avoid with exam-style MCQs at the end.

Most Important String Methods

These are the methods you will use again and again. Learn them by heart:

MethodWhat it doesExample
length()Number of characters"Java".length() -> 4
charAt(i)Character at index i"Java".charAt(1) -> 'a'
toUpperCase() / toLowerCase()Change case"Java".toUpperCase() -> "JAVA"
trim()Remove leading/trailing spaces" hi ".trim() -> "hi"
substring(a, b)Part of string from index a to b-1"Program".substring(3) -> "gram"
replace(a, b)Replace all a with b"a-b".replace("-","") -> "ab"
equals(str)Compare content (case sensitive)"a".equals("a") -> true
equalsIgnoreCase(str)Compare ignoring case"JAVA".equalsIgnoreCase("java") -> true
contains(str)Does it contain the text?"Core Java".contains("Java") -> true
startsWith(s) / endsWith(s)Check beginning/ending"Demo.java".endsWith(".java") -> true
indexOf(ch)First position of the character"Java".indexOf('a') -> 1
split(regex)Break into parts"a,b,c".split(",") -> ["a","b","c"]
toCharArray()Convert to char array"Hi".toCharArray() -> ['H','i']
Example01
JCode Cell
1class StringMethods {
2 public static void main(String[] args) {
3 String s = " Hello Java ";
4 System.out.println(s.trim()); // Hello Java
5 System.out.println(s.length()); // 14
6 System.out.println("Program".substring(3)); // gram
7 System.out.println("Java".toUpperCase()); // JAVA
8 
9 // split a comma separated list
10 String csv = "apple,banana,mango";
11 String[] fruits = csv.split(",");
12 for (String f : fruits) System.out.print(f + " | ");
13 }
14}
Output
Hello Java 14 gram JAVA apple | banana | mango |

== vs equals() — The Five-Object Version

The classic material proves the rule with five comparisons at once. Predict the output before reading:

The lesson: for primitives use ==; for objects use equals(). String overrides equals() to compare content; your own classes must override it too (with hashCode) — see Chapter 15.

Example02
JCode Cell
1class EqDemo {
2 public static void main(String[] args) {
3 int i = 10, j = 10;
4 A a1 = new A();
5 A a2 = new A();
6 String str1 = new String("abc");
7 String str2 = new String("abc");
8 
9 System.out.println(i == j); // true (primitives - values)
10 System.out.println(a1 == a2); // false (different objects)
11 System.out.println(str1 == str2); // false (different objects)
12 System.out.println(a1.equals(a2)); // false (no override -> same as ==)
13 System.out.println(str1.equals(str2)); // true (String overrides equals)
14 }
15}
Output
true false false false true

Quick Practice Questions

Example03
JCode Cell
1// Predict the output:
2String s = "Chocolate";
3System.out.println(s.substring(2, 6));
4System.out.println(s.indexOf('o'));
5System.out.println("abc".equals(new String("abc")));
6StringBuilder sb = new StringBuilder("123");
7sb.append("45");
8System.out.println(sb);
Output
ocol o 3 true 12345
📝 Key Takeaways
  • String is a class, not a primitive; it is immutable.
  • Immutability gives security, thread safety and pool caching.
  • Use StringBuilder (fastest, not thread-safe) or StringBuffer (thread-safe) when text changes often.
  • == compares references; .equals() compares content. Always use .equals() for Strings.
  • Override toString() to print objects meaningfully.
  • Text blocks (Java 15+) make multi-line strings clean.

🧠 Test Your Knowledge

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