added more JUnit tests

Author: S-M-J-I

src/main/java/com/thealgorithms/stacks/GreatestElementConstantTime.java

@@ -37,7 +37,7 @@ public void push(int data) {
         }
 
         mainStack.push(data);
-        if (data > maxStack.peek()) {
+        if (!maxStack.isEmpty() && data > maxStack.peek()) {
             maxStack.push(data);
         }
     }
@@ -55,7 +55,7 @@ public void pop() {
         }
 
         int ele = mainStack.pop();
-        if (ele == maxStack.peek()) {
+        if (!maxStack.isEmpty() && ele == maxStack.peek()) {
             maxStack.pop();
         }
     }

src/main/java/com/thealgorithms/stacks/SmallestElementConstantTime.java

@@ -35,7 +35,7 @@ public void push(int data) {
         }
 
         mainStack.push(data);
-        if (data < minStack.peek()) {
+        if (!minStack.isEmpty() && data < minStack.peek()) {
             minStack.push(data);
         }
     }
@@ -53,7 +53,7 @@ public void pop() {
         }
 
         int ele = mainStack.pop();
-        if (ele == minStack.peek()) {
+        if (!minStack.isEmpty() && ele == minStack.peek()) {
             minStack.pop();
         }
     }

src/test/java/com/thealgorithms/stacks/GreatestElementConstantTimeTest.java

@@ -1,9 +1,12 @@
 package com.thealgorithms.stacks;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+import java.util.NoSuchElementException;
+
+import static org.junit.jupiter.api.Assertions.*;
+
 public class GreatestElementConstantTimeTest {
 
     private GreatestElementConstantTime gect;
@@ -33,4 +36,22 @@ public void testMinTwo() {
         gect.pop();
         assertEquals(10, gect.getMaximumElement());
     }
+
+    @Test
+    public void testNullMax() {
+        gect.push(10);
+        gect.push(20);
+        gect.pop();
+        gect.pop();
+        assertNull(gect.getMaximumElement());
+    }
+
+    @Test
+    public void testBlankHandle() {
+        gect.push(10);
+        gect.push(1);
+        gect.pop();
+        gect.pop();
+        assertThrows(NoSuchElementException.class, () -> gect.pop());
+    }
 }

src/test/java/com/thealgorithms/stacks/SmallestElementConstantTimeTest.java

@@ -1,9 +1,12 @@
 package com.thealgorithms.stacks;
 
-import static org.junit.jupiter.api.Assertions.assertEquals;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+import java.util.NoSuchElementException;
+
+import static org.junit.jupiter.api.Assertions.*;
+
 public class SmallestElementConstantTimeTest {
 
     private SmallestElementConstantTime sect;
@@ -32,4 +35,23 @@ public void testMinTwo() {
         sect.pop();
         assertEquals(5, sect.getMinimumElement());
     }
+
+    @Test
+    public void testNullMin() {
+        sect.push(10);
+        sect.push(20);
+        sect.pop();
+        sect.pop();
+        assertNull(sect.getMinimumElement());
+    }
+
+    @Test
+    public void testBlankHandle() {
+        sect.push(10);
+        sect.push(1);
+        sect.pop();
+        sect.pop();
+        assertThrows(NoSuchElementException.class, () -> sect.pop());
+    }
+
 }