Handbooks / Java / Chapter 2

Strings and Immutability

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Builds on OOP in Java. Next up: Exceptions.

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Immutability and the String Pool: The Mental Model

1. Immutable Strings

Immutable = state fixed after construction
concat, replace, toUpperCase, trim return a NEW String
Original stays untouched: s.trim(); alone is lost
final class, private value: nobody can subclass or mutate
Gives thread-safe use, cached hash (HashMap key), safe paths

2. The String Pool

Pool = JVM-wide table of unique literals, on the heap
Literal reuses the pooled object: "a" == "a" is true
new String("a") = separate heap object, outside the pool
intern() returns the pooled instance

java

String a = "a";
String n = new String("a");
System.out.println(a == n);
System.out.println(n.intern() == a);

output

false
true

3. Constants vs Runtime

Expression	==	Why
`"a" + "b" == "ab"`	true	folded at compile time, pooled
`x + "b" == "ab"` (non-final `x`)	false	runtime concat, new object

Remember

== compares references, equals() compares content. Always assign the result: s = s.trim().

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equals vs == and Comparing Strings

1. == vs equals()

== compares references: same object?
equals() compares content
For Strings you almost always want equals()

java

String s1 = "hi", s2 = "hi";
String s3 = new String("hi");
System.out.println(s1 == s2);
System.out.println(s1 == s3);
System.out.println(s1.equals(s3));

literals share one pooled object; new String makes another

output

true
false
true

2. Null-safe and ordering

Call	Gives	Use when
`"admin".equals(input)`

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StringBuilder and Efficient String Building

1. What StringBuilder Is

Mutable char sequence with an internal resizable buffer
append, insert, delete, reverse, setCharAt change it in place
Each returns this, so calls can be chained

2. Builder vs Concatenation

+= in a loop makes a new String every pass: O(n^2) copying
One builder: append in the loop, toString() once at the end

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Text Blocks

1. Syntax and Indentation

Text block = multi-line literal in """ (Java 15+)
Content starts on the line after the opening """
Common leading whitespace is stripped
Closing """ position counts; move it left to keep indent

java

String json = """
    {"id": 1}
    """;

quotes need no escaping: good for JSON, SQL, HTML

2. Newlines and Escapes

Item	Effect
Line break	kept as `\n`
`\` at line end

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Immutable Classes, Records and Defensive Copies

1. The Immutable Recipe

Declare the class final
All fields private final
Set them only in the constructor
No setters
Never return mutable internals

final is not immutable

final on a field stops reassignment only. A final List or Date can still be changed through its methods.

2. Defensive Copies

Copy in: new ArrayList<>(list), date.clone()
Copy out: List.copyOf, Collections.unmodifiableList

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