## Is Accessing \'array.length\' Really a Performance Bottleneck?

The Performance Cost of Accessing 'array.length'

Q: Is calling 'array.length' more computationally expensive than accessing a regular variable?

In an effort to optimize code by converting for loops to for-each loops, many developers encounter constructs like:

for (int i = 0, n = a.length; i < n; i++) {
    ...
}

where a is an array. Some question whether initializing n with a.length introduces performance penalties compared to omitting n entirely:

for (int i = 0; i < a.length; i++) {
    ...
}

A: No, calling 'array.length' has a constant time (O(1)) cost.

Unlike the size() method of collections, which incurs a performance overhead with each invocation, accessing array.length is equivalent to reading a local variable. This is because array.length is a public final member of the array class and is therefore as fast to access as any other variable.

Optimization by JIT Compilers

Modern Just-In-Time (JIT) compilers often optimize calls to array.length by eliminating them entirely. This is because the compiler can determine that the length of the array remains constant throughout the loop's execution.

Confirmation via Code Analysis

The efficiency of array.length access can be verified by examining the generated native code produced by the JVM's JIT compiler. In most cases, the call to array.length will be optimized out.

Exceptions to the Rule

There are a few scenarios where the JIT compiler may not be able to optimize array.length access:

• Code execution is currently in debug mode.
• The loop body contains a significant number of local variables, causing register spilling.

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