Simplify null checks.
diff --git a/commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/optim/nonlinear/scalar/noderiv/SimplexOptimizer.java b/commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/optim/nonlinear/scalar/noderiv/SimplexOptimizer.java
index 0d310b9..0373ae4 100644
--- a/commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/optim/nonlinear/scalar/noderiv/SimplexOptimizer.java
+++ b/commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/optim/nonlinear/scalar/noderiv/SimplexOptimizer.java
@@ -21,6 +21,7 @@
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.Collections;
+import java.util.Objects;
 import java.util.function.UnaryOperator;
 import java.util.function.IntSupplier;
 import java.util.concurrent.CopyOnWriteArrayList;
@@ -159,9 +160,7 @@
      * @param cb Callback.
      */
     public void addObserver(Observer cb) {
-        if (cb == null) {
-            throw new NullPointerException("Callback");
-        }
+        Objects.requireNonNull(cb, "Callback");
         callbacks.add(cb);
     }
 
@@ -338,12 +337,9 @@
      * was passed to the {@link #optimize(OptimizationData[]) optimize} method.
      */
     private void checkParameters() {
-        if (updateRule == null) {
-            throw new NullPointerException("No update rule");
-        }
-        if (initialSimplex == null) {
-            throw new NullPointerException("No initial simplex");
-        }
+        Objects.requireNonNull(updateRule, "Update rule");
+        Objects.requireNonNull(initialSimplex, "Initial simplex");
+
         if (getLowerBound() != null ||
             getUpperBound() != null) {
             throw new MathUnsupportedOperationException(LocalizedFormats.CONSTRAINT);