The main difference between orElse and orElseGet in the Java Optional class is when and how the default value is computed:

• orElse(T other): The argument is always evaluated, even if the Optional already has a value.
• orElseGet(Supplier<? extends T> otherSupplier): The supplier is only called if the Optional is empty, meaning the logic to create the default value is evaluated lazily, only when needed

Method	Argument Type	When is it evaluated?	Use Case
orElse	T (actual value/result)	Always, even if Optional is present	Use when default value is cheap to create or constant
orElseGet	Supplier<? extends T>	Only if Optional is empty	Use when creation of default is expensive or involves logic

Example

Optional<String> opt = Optional.of("present");

// orElse: computes getDefaultValue() even though not used
opt.orElse(getDefaultValue());

// orElseGet: only computes getDefaultValue() if Optional is empty
opt.orElseGet(() -> getDefaultValue());

If getDefaultValue() prints something, with orElse() you’ll see the print statement executed every time, even when the Optional’s value is present. With orElseGet(), it will only execute if the Optional is empty.

Performance and Best Practices

• Use orElse when the default value is simple or constant (e.g., a literal).
• Use orElseGet when the default value computation is expensive (e.g., database call, heavy calculation), or has side effects, because it avoids unnecessary work.

In summary, prefer orElseGet for lazy/economical computation of fallbacks, orElse for cheap constants. Using orElse with expensive or side-effecting defaults can negatively impact your app’s performance or behavior