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Java Platform, Standard Edition Nashorn User's Guide
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# 2 The Nashorn Java API

This chapter describes how to access Java from a script interpreted by the Nashorn engine.

The sections in this chapter contain examples of script statements interpreted by the Nashorn engine in interactive language shell mode. This interactive shell is started by running the `jjs` command without any scripts passed to it. This is useful for trying things out, but the main purpose of the Nashorn Java API is to write Java applications as scripts that can be interpreted by the Nashorn engine.

## 2.1 Accessing Java Classes

There are two approaches to access packages and classes using Nashorn: the traditional approach is to use the `Packages` global object, and the recommended approach is to use the `Java` global object. This section describes both approaches.

The predefined top-level `Packages` object enables you to access Java packages and classes using their fully qualified names, as if they are properties of the `Packages` object. The following example shows how you can access the `MyPackage` package and its `MyClass` class if `MyPackage.jar` is in your class path:

    jjs> Packages.MyPackage
    [JavaPackage MyPackage]
    jjs> Packages.MyPackage.MyClass
    [JavaClass MyPackage.MyClass]

Accessing standard Java packages and classes is more straightforward than accessing custom packages and classes. For your convenience, there are global objects defined for each of the standard Java packages: `com`, `edu`, `java`, `javafx`, `javax`, and `org`. They have aliases that correspond to properties of the `Packages` object. The following example shows how you can access the `java.lang` package and the `java.lang.System` class:

    jjs> java.lang
    [JavaPackage java.lang]
    jjs> typeof java.lang
    object
    jjs> java.lang.System
    [JavaClass java.lang.System]
    jjs> typeof java.lang.System
    function

As you can see from the previous example, Nashorn interprets Java packages as `JavaPackage` objects, and Java classes as `JavaClass` function objects, which can be used as constructors for the classes. For more information about instantiating a class, see Section 2.2, "Creating Java Objects".

The traditional approach for accessing Java packages and classes is intuitive and straightforward, but at the same time, it can be inefficient, limited, and error-prone for the following reasons:

- Each property access has a cost, so accessing a package or class in a deep hierarchy can be slow.

- There is no special syntax for creating Java arrays. You must use the `java.lang.reflect.Array` class as a workaround.

- If you misspell a class name, Nashorn assumes that you provided a package name, and interprets it as a `JavaPackage` object instead of a `JavaClass` function object. You might not be aware of this until an error is thrown when you attempt to use it as a class. To avoid this, use the `typeof` operator to conditionally test that the construct you are trying to access is interpreted as a function object. The following example shows how this conditional check works:

      jjs> typeof java.lang.System == "function"
      true
      jjs> typeof java.lang.Zyztem == "function"
      false

To avoid the disadvantages of the approach previously described, Nashorn defines the `Java` global object that has several functions for working with Java classes. The `Java.type()` function takes a string with the fully qualified Java class name, and returns the corresponding `JavaClass` function object. The following example shows how you can access the `java.lang.System` class:

    jjs> Java.type("java.lang.System")
    [JavaClass java.lang.System]

Similar to importing classes in Java, it is a good practice to declare variables of `JavaClass` type at the beginning of a script. The following example shows how you can declare the `System` variable and give it a value of the `java.lang.System` class:

    jjs> var System = Java.type("java.lang.System")
    jjs> System
    [JavaClass java.lang.System]

## 2.2 Creating Java Objects

To instantiate a class, pass the `JavaClass` function object to the `new` operator. Nashorn invokes the corresponding constructor based on the arguments passed to the function. The following example shows how you can instantiate the `java.util.HashMap` class with the default initial capacity and with the initial capacity set to 100:

    jjs> var HashMap = Java.type("java.util.HashMap")
    jjs> var mapDef = new HashMap()
    jjs> var map100 = new HashMap(100)

## 2.3 Accessing Class and Instance Members

You can use the standard dot notation to access static fields, methods, and inner classes as follows:

    jjs> Java.type("java.lang.Math").PI
    3.141592653589793
    jjs> Java.type("java.lang.System").currentTimeMillis()
    1375813353330
    jjs> Java.type("java.util.Map").Entry
    [JavaClass java.util.Map$Entry]

An inner class can also be accessed using internal representation with the dollar sign (`$`) as the separator, or a dot, which is consistent with Java:

    jjs> Java.type("java.util.Map$Entry")
    [JavaClass java.util.Map$Entry]
    jjs> Java.type("java.util.Map.Entry")
    [JavaClass java.util.Map$Entry]

To invoke an instance method or access an instance field of an object, use the dot operator, similar to how it is done in Java. The following example shows how you can call the `toUpperCase()` method on a `String` object:

    jjs> var String = Java.type("java.lang.String")
    jjs> var str = new String("Hello")
    jjs> str
    Hello
    jjs> var upper = str.toUpperCase()
    jjs> upper
    HELLO

Nashorn also supports member access using the bracket notation, where you specify the name of the member as a string between brackets (`[]`) that immediately follow the class (in case of a static member) or object (in case of an instance member). This method is defined by the ECMAScript as an alternative to the dot notation, and is not intuitive for Java developers. However, it can be used to resolve method overload ambiguity. By default, Nashorn uses the overloaded method that best matches the arguments, and this is not always what you expect. For example, if you want to print a `double` value, you must use the `java.lang.System.out.println(double)` method overload, as shown in the following example:

    jjs> Java.type("java.lang.System").out.println(10)
    10
    jjs> Java.type("java.lang.System").out["println(double)"](10)
    10.0

## 2.4 Using JavaBeans

Nashorn enables you to treat accessor and mutator methods in JavaBeans as equivalent JavaScript properties. The name of the property is the name of the JavaBean method without the `get` or `set` suffix, and starts with a lowecase letter. For example you can call the `getYear()` and `setYear()` methods in a `java.util.Date` object using the `year` property as follows:

    jjs> var Date = Java.type("java.util.Date")
    jjs> var date = new Date()
    jjs> date.year + 1900
    2013
    jjs> date.year = 2014 - 1900
    114
    jjs> date.year + 1900
    2014

## 2.5 Working with Java Arrays

To access a Java array class, pass to the `Java.type()` function the type of objects that comprise the array followed by a pair of brackets (similar to Java syntax). The following example shows how you can access a Java array of integers and a Java array of `String` objects:

    jjs> Java.type("int[]")
    [JavaClass [I]
    jjs> Java.type("java.lang.String[]")
    [JavaClass [Ljava.lang.String;]

After you have the array type object, you can use it to instantiate an array as you do any other class. You can access array entries by their indexes, and use the dot or bracket notation to access members (similar to Java syntax), as shown in the following example:

    jjs> var IntArrayType = Java.type("int[]")
    jjs> var arr = new IntArrayType(10)
    jjs> arr[1] = 123
    123
    jjs> arr[2] = 321
    321
    jjs> arr[1] + arr[2]
    444
    jjs> arr[10]
    java.lang.ArrayIndexOutOfBoundsException: Array index out of range: 10
    jjs> arr.length
    10

If you have an existing JavaScript array, you can convert it to a Java array using the `Java.to()` function. The following example shows how you can convert a JavaScript array of strings `"a"`, `"b"`, and `"c"`, to a `java.lang.String[]` array with the same values:

    jjs> var jsArr = ["a","b","c"]
    jjs> var strArrType = Java.type("java.lang.String[]")
    jjs> var javaArr = Java.to(jsArr, strArrType)
    jjs> javaArr.class
    class [Ljava.lang.String;
    jjs> javaArr[0]
    a

You can iterate through a Java array's indexes and values using the `for` and `for each` statements as follows:

    jjs> for (var i in javaArr) print(i)
    0
    1
    2
    jjs> for each (var i in javaArr) print(i)
    a
    b
    c

## 2.6 Working with Java Strings

Strings are interpreted by Nashorn as `java.lang.String` objects. However, if you concatenate two strings you get an instance of the `jdk.nashorn.internal.runtime.ConsString` class. This is not a problem for most scripts, because both classes implement the `java.lang.CharSequence` interface, but in some cases, Nashorn passes the `ConsString` object when a method expects a `java.lang.Object` argument. To avoid this, use the `String()` function to ensure that the string is a Java `String` object, as shown in the following example:

    jjs> var a = "abc"
    jjs> a.class
    class java.lang.String
    jjs> var b = a + "def"
    jjs> b.class
    class jdk.nashorn.internal.runtime.ConsString
    jjs> var c = String(b)
    jjs> c.class
    class java.lang.String

## 2.7 Working with Java Numbers

Nashorn interprets numbers as `java.lang.Double`, `java.lang.Long`, or `java.lang.Integer` objects, depending on the computation performed. You can use the `Number()` function to force a number to be a `Double` object, as shown in the following example:

    jjs> var intNum = 10
    jjs> intNum.class
    class java.lang.Integer
    jjs> var dblNum = Number(intNum)
    jjs> dblNum.class
    class java.lang.Double

## 2.8 Working with Java Collections

Nashorn interprets Java collections as arrays. You can access collection elements using the index in brackets (`[]`) and iterate over the values of a collection using the `for each` statement, as shown in the following example:

    jjs> var ArrayList = Java.type("java.util.ArrayList")
    jjs> var alist = new ArrayList()
    jjs> alist.add("a")
    true
    jjs> alist.add("b")
    true
    jjs> alist.add("c")
    true
    jjs> alist[1]
    b
    jjs> for each (var i in alist) print(i)
    a
    b
    c

Unlike other collections, to iterate over keys and values in a map, you must use the `keySet()` and `values()` methods. The following example shows how you can create a `HashMap` object and iterate over its keys and values:

    jjs> var HashMap = Java.type("java.util.HashMap")
    jjs> var hm = new HashMap()
    jjs> hm.put("name", "Bob")
    jjs> hm.put("age", 40)
    jjs> hm.put("weight", 180)
    jjs> for each (var i in hm.keySet()) print(i)
    weight
    age
    name
    jjs> for each (var i in hm.values()) print(i)
    180
    40
    Bob

## 2.9 Extending Java Classes

You can extend a class using the `Java.extend()` function that takes a Java type as the first argument and method implementations (in the form of JavaScript functions) as the other arguments. Example 2-1 shows a script that extends the `java.lang.Runnable` interface and uses it to construct a new `java.lang.Thread` object.

Example 2-1 Extending a Java Class

    var Run = Java.type("java.lang.Runnable");
    var MyRun = Java.extend(Run, {
        run: function() {
            print("Run in separate thread");
        }
    });
    var Thread = Java.type("java.lang.Thread");
    var th = new Thread(new MyRun());

Nashorn can automatically extend single abstract method (SAM) classes if you provide the function for implementing the method as the argument to the constructor. Example 2-2 shows a script that extends the `java.lang.Runnable` interface and uses it to construct a new `java.lang.Thread` object, but it uses fewer lines of code than in Example 2-1, because the `Java.extend()` function is called automatically for a SAM class.

Example 2-2 Extending a Java SAM Class

    var Thread = Java.type("java.lang.Thread")
    var th = new Thread(function() print("Run in a separate thread"))

For more information about the capabilities of the `Java.extend()` function, see Java Scripting Programmer's Guide at `http://docs.oracle.com/javase/8/docs/technotes/guides/scripting/index.html`

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