Arrays.setAll

As explained in 100 Java Mistakes and How to Avoid Them

Arrays.setAll is a Java API operation that fills an array by computing each element from its index with a supplied function.

Arrays.setAll fills every position in an array by calling a supplied function with that position's index and storing the returned value. The index can drive computation or be deliberately ignored. section 2.13 section 3.5

A method reference can select the wrong constructor

StringBuilder[] array = new StringBuilder[size]; Arrays.setAll(array, StringBuilder::new); This looks like a request for an empty StringBuilder at each position. However, the function receives the index, and overload resolution can select StringBuilder(int). The result is equivalent to index -> new StringBuilder(index), so later elements preallocate increasingly large buffers. Large arrays can become extremely slow or fail with OutOfMemoryError. Ignore the index explicitly when no initial capacity is intended: Arrays.setAll(array, index -> new StringBuilder()); section 2.13

Avoid common indexing and method-reference mistakes by applying these checks:

• Use a lambda when a constructor or method reference has overloads or variable-arity parameters; make ignored arguments visible. section 2.13
• Use the lambda index when each output depends on its corresponding input position, such as mapping types[i] into parameterTypes[i]. section 3.5
• Treat constant-index diagnostics inside loops as review prompts rather than proof of an error; rewrite with Arrays.setAll or Stream API when appropriate. section 3.5

Figure 2.9 Final memory layout after invoking Arrays.setAll() for size = 100. The internal byte[] buffer held by StringBuilder becomes larger and larger toward the end of the array.

Inspect how the mistaken constructor choice makes StringBuilder's internal buffers grow toward the end of the array. section 2.13

How 100 Java Mistakes and How to Avoid Them covers it

How deep it goes

Explains the generator signature, overload-resolution hazard, memory consequence, and safer lambda form without covering implementation internals.

Examples

Uses StringBuilder allocation and reflective type-array conversion to demonstrate ignored and required indices.

What it assumes

Assumes Java arrays, lambdas, method references, overloaded constructors, and basic static-analysis terminology.

What you can do after

Apply it via Java's Arrays.setAll API while avoiding accidental constructor arguments and constant-index mistakes.

How it connects

Connects Arrays.setAll with method references, lambda expressions, constructor overloads, static analysis, and Stream API.

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Related concepts

• method referenceused by · A method reference is an expression that denotes an existing method or constructor as a callable functional value.
• indexinguses · Indexing is the organization or use of positions, keys, or lookup structures to identify and retrieve selected data or elements.
• lambdauses · A lambda is a language construct that defines an unnamed function or callable, often as a value.

This explainer was generated by Manning's knowledge graph from the text of 100 Java Mistakes and How to Avoid Them. The section links point to the parts of the book it draws on.