# Phone Directory using TreeMap

### Objective

In this lab, we will create a basic PhoneDirectory by using a TreeMap to store directory entries.

### Learning Objective

By the end of this lesson, learners will be able to utilize the TreeMap.

### Scenario

* Finds the phone number, if any, for a given name and returns the phone number associated with the name if the name does not occur in the phone directory, and then returns the value as null.
* Associates a given name with a given phone number. If the name already exists in the phone directory, the new number replaces the old one. Otherwise, a new name/number pair is added. The name and number should both be non-null. An IllegalArgumentException is thrown if this is not the case.

### Instructions

#### Step 1

Create a new Java project and create a new Class named “PhoneDirectoryWithTreeMap,” and then write the below code in the class.

```java
import java.util.TreeMap;
import java.util.Map;
public class PhoneDirectoryWithTreeMap {
   /**
    * The TreeMap that will store the data.  Both key and value are
    * of type String.  The key represents a name and the value represents
    * the associated phone number.
    */
   private TreeMap<String,String> data;

   /**
    * Constructor creates an initially empty directory.
    */
   public PhoneDirectoryWithTreeMap() {
       this.data = new TreeMap<String,String>();
   }

   /**
    * Finds the phone number, if any, for a given name.
    * @return The phone number associated with the name; if the name does
    *    not occur in the phone directory, then the return value is null.
    */
   public String getNumber( String name ) {
       return   this.data.get(name);
   }

   /**
    * Associates a given name with a given phone number.  If the name
    * already exists in the phone directory, then the new number replaces
    * the old one.  Otherwise, a new name/number pair is added.  The
    * name and number should both be non-null.  An IllegalArgumentException
    * is thrown if this is not the case.    */
   public void putNumber( String name, String number ) {
       if (name == null || number == null)
           throw new IllegalArgumentException("name and number cannot be null");
       this.data.put(name,number);
   }

   /**  Write the contents of the phone directory to System.out.
    */
   public void print() {
       for ( Map.Entry<String,String> entry :   this.data.entrySet() )
           System.out.println( entry.getKey() + ":  " + entry.getValue() );
   }

} // end class PhoneDirectoryWithTreeMap

```

#### Step 2

Create a new Java project and create a new Class named “TestPhoneDirectoryWithTreeMap,” and then write the code below in the class.

```java
public class TestPhoneDirectoryWithTreeMap {
   public static void main(String[] args) {
       PhoneDirectoryWithTreeMap directory = new PhoneDirectoryWithTreeMap();
       System.out.println("This program creates a PhoneDirectoryWithTreeMap and");
       System.out.println("adds several entries.  It then prints the contents of");
       System.out.println("directory and does a few lookups.");
       System.out.println();
       directory.putNumber("Fred","555-1234");
       directory.putNumber("Barney","555-2345");
       directory.putNumber("Wilma","555-3456");
       directory.putNumber("Wilma","555-3456");
       directory.putNumber("Wilma","555-2563");
      // directory.putNumber("James",null);
       //directory.putNumber(null,"555-2853");

       System.out.println("Contents are:");
       System.out.println();
       directory.print();
       System.out.println();
       System.out.println("Number for Fred is " + directory.getNumber("Fred"));
       System.out.println("Number for Wilma is " + directory.getNumber("Wilma"));
       System.out.println("Number for Tom is " + directory.getNumber("Tom"));
       System.out.println("Number for Tom is " + directory.getNumber("Alex"));
       // The output from the last line should be null.
   }
}

```

#### Output

```
This program creates a PhoneDirectoryWithTreeMap and
adds several entries; it then prints the contents of
the directory and performs a few lookups.

Contents are:

Barney:  555-2345
Fred:  555-1234
Wilma:  555-2563

Number for Fred is 555-1234
Number for Wilma is 555-2563
Number for Tom is null
Number for Tom is null
```

{% file src="/files/KQJvjSKj2CK12YONEWiG" %}


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://peterlulu666.gitbook.io/tek-system-java-developer/week-3/lab/phone-directory-using-treemap.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.