A DESCRIPTION OF THE REQUEST :
SingletonMap have single mapping and should check for existing key before throw exception:

  @Override
  public V computeIfAbsent(K key,
          Function<? super K, ? extends V> mappingFunction) {
      if (eq(key, k))
          return v;
      throw new UnsupportedOperationException();
  }

Default method from java.util.Map works as expected except mapping function was called.

JUSTIFICATION :
Changes in underlying framework which starts using computeIfAbsent cause code changes to avoid SingletonMap

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
method return value
ACTUAL -
java.lang.UnsupportedOperationException thrown

---------- BEGIN SOURCE ----------
java.util.Collections.singletonMap("k", "v").computeIfAbsent("k", k -> "v" );
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
use another Map implementation