Is the Sum of Two Calls to System.nanoTime() Always Non-negative in Java?

Is calculating the sum of two System.nanoTime() calls guaranteed to be greater than or equal to zero in Java?

long startTime = System.nanoTime();
long endTime = System.nanoTime();
long totalTime = startTime + endTime;

In Java, the method System.nanoTime() returns the current value of the most precise available system timer, in nanoseconds. While it generally returns a non-negative value when measuring elapsed time, the question arises whether the sum of two calls to this method can ever yield a negative result. This answer breaks down the behavior of System.nanoTime() and examines the conditions under which its output can be evaluated.

long startTime = System.nanoTime();
// Perform some operations
long endTime = System.nanoTime();
long totalTime = startTime + endTime;
// Ensure totalTime remains non-negative as long as nanotime is consistent.

Causes

• System.nanoTime() returns a long integer that represents the current value of the running Java Virtual Machine timer, typically giving a value larger than 0.
• The return value of System.nanoTime() can only decrease due to system clock adjustments, although this is a rare event.

Solutions

• The addition of two long integers in Java will not overflow under normal conditions due to how System.nanoTime() values should behave.
• To assure non-negativity, avoid using System.nanoTime() directly in critical calculations where clock adjustments can briefly change the retrieved values.

Mistake: Assuming that System.nanoTime() cannot return a negative value under any circumstances.

Solution: While System.nanoTime() is typically non-negative, understand that it can appear to drop when the system clock is adjusted. However, its rate of increment is consistent unless the system time base is altered.

Mistake: Using System.nanoTime() for benchmarking without considering overhead.

Solution: To accurately measure elapsed time, capture the start and end time around the code segment you're benchmarking to minimize external influences on the timing.