Java · Theory

Substring in Java

Theory

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Substring in Java

Substring is about working with text in Java. Remember the base rule first: String is immutable. Compare content with equals, not ==.

For Substring, common APIs from memory: length(), charAt(i), substring(start, end), contains, equalsIgnoreCase, toUpperCase. Index starts at 0.

If Substring is StringBuilder/StringBuffer, you change text in place with append. If it is comparison, show == false and equals true for two new String("Hi").

For Substring, say the output before you code. Off-by-one on substring is a common trap (end index is exclusive).

Let's take this on the board with one tiny Main class — no extra files. Substring in Java — shows length, substring, case change, and equalsIgnoreCase. Start from main, go line by line, and stop at each print. That output is the proof for Substring.

Exam tip

== vs equals() on String is a classic trap.

Example

// String methods
public class Main {
  public static void main(String[] args) {
    String s = "PrepPilot";
    System.out.println(s.length());
    System.out.println(s.substring(0, 4));
    System.out.println(s.toUpperCase());
    System.out.println("prep".equalsIgnoreCase("PREP"));
  }
}

Substring in Java — shows length, substring, case change, and equalsIgnoreCase.

Short notes

DefSubstring works with Java text. String itself is immutable.
RuleSubstring — compare content with equals, not ==.
RememberSubstring — length, charAt, substring. substring end index is exclusive.
TrapSubstring — == compares references. User input almost never matches with ==.

// String methods public class Main { public static void main(String[] args) { String s = "PrepPilot"; System.out.println(s.length()); System.out.println(s.substring(0, 4)); System.out.println(s.toUpperCase()); System.out.println("prep".equalsIgnoreCase("PREP")); } }

Substring in Java

Example

Short notes

Questions

Substring in Java

Example

Short notes

Questions