Do Step1: method test

src/test/java/method/CharAtTest.java

@@ -0,0 +1,31 @@
+package method;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class CharAtTest {
+    @ParameterizedTest
+    @CsvSource(value = {"0:a", "1:b", "2:c"}, delimiter = ':')
+    @DisplayName("test charAt() with in of bound")
+    public void charAt(int input, char expected) {
+        String abc = "abc";
+        assertEquals(expected, abc.charAt(input));
+    }
+
+    @Test
+    @DisplayName("test charAt() with out of bound")
+    public void charAtException() {
+        String abc = "abc";
+
+        assertThatThrownBy(() -> {
+            System.out.println(abc.charAt(3));
+        })
+                .isInstanceOf(IndexOutOfBoundsException.class)
+                .hasMessageContaining("String index out of range: 3");
+    }
+}

src/test/java/method/SetTest.java

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+package method;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class SetTest {
+    private Set<Integer> numbers;
+
+    @BeforeEach
+    void setUp() {
+        numbers = new HashSet<>();
+        numbers.add(1);
+        numbers.add(1);
+        numbers.add(2);
+        numbers.add(3);
+    }
+
+    @Test
+    @DisplayName("test size()")
+    public void size() {
+        assertThat(numbers.size()).isEqualTo(3);
+    }
+
+    @ParameterizedTest
+    @CsvSource(value = {
+            "1:true",
+            "2:true",
+            "3:true",
+            "4:false",
+            "5:false",
+    }, delimiter = ':')
+    @DisplayName("test contains()")
+    public void contains(int input, boolean expected) {
+        assertEquals(expected, numbers.contains(input));
+    }
+}

src/test/java/method/SplitTest.java

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+package method;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Java6Assertions.assertThat;
+
+public class SplitTest {
+    @Test
+    @DisplayName("test split()")
+    public void splitTest() {
+        String testString = "1,2";
+        String[] splitted = testString.split(",");
+
+        assertThat(splitted).contains("1");
+        assertThat(splitted).contains("2");
+        assertThat(splitted).containsExactly("1", "2");
+    }
+}

src/test/java/method/SubstringTest.java

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+package method;
+
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class SubstringTest {
+    @Test
+    @DisplayName("test substring()")
+    public void substringTest() {
+        String testString ="(1,2)";
+        String testSubString = testString.substring(1, 4);
+
+        assertThat(testSubString).isEqualTo("1,2");
+    }
+}