diff --git a/src/main/java/Length.java b/src/main/java/Length.java
index dbc9d2c..deed865 100644
--- a/src/main/java/Length.java
+++ b/src/main/java/Length.java
@@ -1,35 +1,35 @@
 public class Length {
     private final double value;
-    private final String unit;
+    private final Unit unit;
 
-    public Length(double val, String uinnt) {
+    public Length(double val, Unit unit) {
         this.value = val;
-        this.unit = uinnt;
+        this.unit = unit;
     }
 
-    public Length as(String u) {
+    public Length as(Unit unit) {
         Length len = this;
-        if (this.unit.equals("f")) {
-            if (u.equals("yard")) {
-                len = new Length(this.value / 3, u);
-            } else if (u.equals("inch")) {
-                len = new Length(this.value * 12, u);
+        if (this.unit ==Unit.FOOT) {
+            if (unit == Unit.YARD) {
+                len = new Length(this.value / 3, unit);
+            } else if (unit == Unit.INCH) {
+                len = new Length(this.value * 12, unit);
             }
         }
 
-        if (this.unit.equals("yard")) {
-            if (u.equals("inch")) {
-                len = new Length(this.value * 36, u);
-            } else if (u.equals("f")){
-                len = new Length(this.value * 3, u);
+        if (this.unit ==Unit.YARD) {
+            if (unit == Unit.INCH) {
+                len = new Length(this.value * 36, unit);
+            } else if (unit == Unit.FOOT) {
+                len = new Length(this.value * 3, unit);
             }
         }
 
-        if (this.unit.equals("inch")) {
-            if (u.equals("f")) {
-                len = new Length(this.value / 12, u);
-            } else if (u.equals("yard")) {
-                len = new Length(this.value / 36, u);
+        if (this.unit ==Unit.INCH) {
+            if (unit == Unit.FOOT) {
+                len = new Length(this.value / 12, unit);
+            } else if (unit == Unit.YARD) {
+                len = new Length(this.value / 36, unit);
             }
         }
 
@@ -40,7 +40,7 @@ public double getVal() {
         return this.value;
     }
 
-    public String getUinnt() {
+    public Unit getUnit() {
         return this.unit;
     }
 }
diff --git a/src/main/java/Unit.java b/src/main/java/Unit.java
new file mode 100644
index 0000000..5726bc4
--- /dev/null
+++ b/src/main/java/Unit.java
@@ -0,0 +1,9 @@
+/**
+ * Created with IntelliJ IDEA.
+ * User: lai.yi
+ * Date: 2020/2/1
+ * Description:
+ **/
+public enum Unit {
+    FOOT, INCH, YARD
+}
diff --git a/src/test/java/LengthTest.java b/src/test/java/LengthTest.java
index 4cc0cb8..271ae5b 100644
--- a/src/test/java/LengthTest.java
+++ b/src/test/java/LengthTest.java
@@ -6,89 +6,89 @@
 public class LengthTest {
     @Test
     public void should_1_inch_equals_1_inch() {
-        Length result = new Length(1, "inch").as("inch");
+        Length result = new Length(1, Unit.INCH).as(Unit.INCH);
 
         assertThat(result.getVal(), is(1.0));
-        assertThat(result.getUinnt(), is("inch"));
+        assertThat(result.getUnit(), is(Unit.INCH));
     }
 
     @Test
     public void should_2_feet_equals_2_feet() {
-        Length result = new Length(2, "f").as("f");
+        Length result = new Length(2, Unit.FOOT).as(Unit.FOOT);
 
         assertThat(result.getVal(), is(2.0));
-        assertThat(result.getUinnt(), is("f"));
+        assertThat(result.getUnit(), is(Unit.FOOT));
     }
 
     @Test
     public void should_1_yard_equals_1_yard() {
-        Length result = new Length(1, "yard").as("yard");
+        Length result = new Length(1, Unit.YARD).as(Unit.YARD);
 
         assertThat(result.getVal(), is(1.0));
-        assertThat(result.getUinnt(), is("yard"));
+        assertThat(result.getUnit(), is(Unit.YARD));
     }
 
     @Test
     public void should_1_foot_equals_12_inches() {
-        Length result = new Length(1, "f").as("inch");
+        Length result = new Length(1, Unit.FOOT).as(Unit.INCH);
 
         assertThat(result.getVal(), is(12.0));
-        assertThat(result.getUinnt(), is("inch"));
+        assertThat(result.getUnit(), is(Unit.INCH));
     }
 
     @Test
     public void should_3_foot_equals_1_yard() {
-        Length result = new Length(3, "f").as("yard");
+        Length result = new Length(3, Unit.FOOT).as(Unit.YARD);
 
         assertThat(result.getVal(), is(1.0));
-        assertThat(result.getUinnt(), is("yard"));
+        assertThat(result.getUnit(), is(Unit.YARD));
     }
 
     @Test
     public void should_1_yard_equals_3_feet() {
-        Length result = new Length(1, "yard").as("f");
+        Length result = new Length(1, Unit.YARD).as(Unit.FOOT);
 
         assertThat(result.getVal(), is(3.0));
-        assertThat(result.getUinnt(), is("f"));
+        assertThat(result.getUnit(), is(Unit.FOOT));
     }
 
     @Test
     public void should_1_yard_equals_36_inches() {
-        Length result = new Length(1, "yard").as("inch");
+        Length result = new Length(1, Unit.YARD).as(Unit.INCH);
 
         assertThat(result.getVal(), is(36.0));
-        assertThat(result.getUinnt(), is("inch"));
+        assertThat(result.getUnit(), is(Unit.INCH));
     }
 
     @Test
     public void should_2_yards_equals_72_inches() {
-        Length result = new Length(2, "yard").as("inch");
+        Length result = new Length(2, Unit.YARD).as(Unit.INCH);
 
         assertThat(result.getVal(), is(72.0));
-        assertThat(result.getUinnt(), is("inch"));
+        assertThat(result.getUnit(), is(Unit.INCH));
     }
 
     @Test
     public void should_12_inches_equals_1_foot() {
-        Length result = new Length(12, "inch").as("f");
+        Length result = new Length(12, Unit.INCH).as(Unit.FOOT);
 
         assertThat(result.getVal(), is(1.0));
-        assertThat(result.getUinnt(), is("f"));
+        assertThat(result.getUnit(), is(Unit.FOOT));
     }
 
     @Test
     public void should_36_inches_equals_1_yard() {
-        Length result = new Length(36, "inch").as("yard");
+        Length result = new Length(36, Unit.INCH).as(Unit.YARD);
 
         assertThat(result.getVal(), is(1.0));
-        assertThat(result.getUinnt(), is("yard"));
+        assertThat(result.getUnit(), is(Unit.YARD));
     }
 
     @Test
     public void should_18_inches_equals_half_yard() {
-        Length result = new Length(18, "inch").as("yard");
+        Length result = new Length(18, Unit.INCH).as(Unit.YARD);
 
         assertThat(result.getVal(), is(0.5));
-        assertThat(result.getUinnt(), is("yard"));
+        assertThat(result.getUnit(), is(Unit.YARD));
     }
 }