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Strings: immutability, equality, common methods

A String is an immutable piece of text: no method ever changes it, every "change" returns a new String.

๐Ÿ”Ž Unravel this example

At a glance

Mental model

A printed label.

What happens, in order
  1. 1Common methods: length(), charAt(i), substring(from, to), indexOf("x"), contains, startsWith, replace, trim/strip, split, toUpperCase, isEmpty and isBlank (true for only spaces)
  2. 2== asks "is it the same object?", equals asks "is the text the same?"String literals are shared through a pool, so "a" == "a" can be true while new String("a") == "a" is false. Always use equals; for a possible null write "yes".equals(input).
  3. 3Positions start at 0 and ranges are half open: "hello".substring(1, 3) is "el"
  4. 4Gluing many pieces in a loop with + creates a new String every timeUse a StringBuilder for that.
Remember
Watch out
Comparing with ==Expecting s.toUpperCase(); alone to change sUsing split(".") or split("|") without escapingConcatenating in a big loop
Example (highlighted lines explain it)
public class Names {  static String initials(String full) {    if (full == null || full.isBlank()) return "";    StringBuilder sb = new StringBuilder();    for (String part : full.strip().split("\\s+")) sb.append(part.charAt(0));    return sb.toString().toUpperCase();  }  static boolean sameUser(String a, String b) { return a.equalsIgnoreCase(b.substring(1)); }}
  1. line 2 uses String, the core of Strings: immutability, equality, common methods
  2. line 3 uses isBlank, the core of Strings: immutability, equality, common methods
  3. line 4 uses String, the core of Strings: immutability, equality, common methods
  4. line 5 uses String, the core of Strings: immutability, equality, common methods
  5. line 6 uses String, the core of Strings: immutability, equality, common methods
  6. line 8 uses String, the core of Strings: immutability, equality, common methods
Read the full explanation (about 1 min)

A String is an immutable piece of text: no method ever changes it, every "change" returns a new String. Compare strings with equals, not ==.

A mental model

A printed label. To change the wording you print a new label; the old one stays as it was, which is why many parts of a program can safely share the same label.

How it works

Common methods: length(), charAt(i), substring(from, to), indexOf("x"), contains, startsWith, replace, trim/strip, split, toUpperCase, isEmpty and isBlank (true for only spaces).

== asks "is it the same object?", equals asks "is the text the same?". String literals are shared through a pool, so "a" == "a" can be true while new String("a") == "a" is false. Always use equals; for a possible null write "yes".equals(input).

Positions start at 0 and ranges are half open: "hello".substring(1, 3) is "el".

Gluing many pieces in a loop with + creates a new String every time. Use a StringBuilder for that.

Going deeper

split takes a regular expression, so "a.b".split(".") returns nothing useful: the dot matches every character. Use split("\\.") or Pattern.quote.

Case-insensitive comparison: equalsIgnoreCase. For user-visible sorting or case changes in different languages, specify a Locale (toLowerCase(Locale.ROOT)): the Turkish dotless i breaks naive code.

Strings are used as map keys because their hashCode is cached after first use and they cannot change under the map. Keep passwords in char[] instead if you want to wipe them.

Under the hood

A String holds a byte[] plus a coder flag (compact strings: one byte per character when all characters are Latin-1, two otherwise). intern() moves a string into the JVM-wide pool; literals are interned automatically, and substring copies the data in modern Java.

Common mistakes

  • Comparing with ==.
  • Expecting s.toUpperCase(); alone to change s.
  • Using split(".") or split("|") without escaping.
  • Concatenating in a big loop.

How Unravel recognises it

When you paste code, Unravel looks for these signs of strings: immutability, equality, common methods. Spotting them yourself is a good habit when you read unfamiliar code:

Part of a learning path

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Builds on this

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Frequently asked questions

What is Strings: immutability, equality, common methods?

A String is an immutable piece of text: no method ever changes it, every "change" returns a new String. Compare strings with equals, not ==.

What are the most common mistakes with Strings: immutability, equality, common methods?

Comparing with ==. Expecting s.toUpperCase(); alone to change s. Using split(".") or split("|") without escaping. Concatenating in a big loop.

How can I see Strings: immutability, equality, common methods in real code?

Paste your own Java into Unravel and it will point out where strings: immutability, equality, common methods appears, explain how it works at the detail level you choose, and draw the flow of the code around it.