下面這行代碼如何在沒有new運算符的情況下創建對象？

Question

我學到的東西是新建對象是強制性的。所以，這行代碼（來自java swing）是如何工作的？

GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment(); 

Answer 1

這是因爲static實施getLocalGraphicsEnvironment方法，它返回一個GraphicsEnvironment型的。從class itself, definition爲 -

/** 
* Returns the local <code>GraphicsEnvironment</code>. 
* @return the local <code>GraphicsEnvironment</code> 
*/ 
public static synchronized GraphicsEnvironment getLocalGraphicsEnvironment() { 
    if (localEnv == null) { 
     localEnv = createGE(); 
    } 

    return localEnv; 
} 

其中localEnv被聲明爲 -

private static GraphicsEnvironment localEnv; 

Answer 2

下面是從createGE()代碼：

private static GraphicsEnvironment createGE() { 
    GraphicsEnvironment ge; 
    String nm = AccessController.doPrivileged(new GetPropertyAction("java.awt.graphicsenv", null)); 
    try { 
     // long t0 = System.currentTimeMillis(); 
     Class<GraphicsEnvironment> geCls; 
     try { 
      // First we try if the bootclassloader finds the requested 
      // class. This way we can avoid to run in a privileged block. 
      geCls = (Class<GraphicsEnvironment>)Class.forName(nm); 
     } catch (ClassNotFoundException ex) { 
      // If the bootclassloader fails, we try again with the 
      // application classloader. 
      ClassLoader cl = ClassLoader.getSystemClassLoader(); 
      geCls = (Class<GraphicsEnvironment>)Class.forName(nm, true, cl); 
     } 
     ge = geCls.newInstance(); 
     // long t1 = System.currentTimeMillis(); 
     // System.out.println("GE creation took " + (t1-t0)+ "ms."); 
     if (isHeadless()) { 
      ge = new HeadlessGraphicsEnvironment(ge); 
     } 
    } catch (ClassNotFoundException e) { 
     throw new Error("Could not find class: "+nm); 
    } catch (InstantiationException e) { 
     throw new Error("Could not instantiate Graphics Environment: " 
         + nm); 
    } catch (IllegalAccessException e) { 
     throw new Error ("Could not access Graphics Environment: " 
         + nm); 
    } 
    return ge; 
} 

雖然它從未真正直接調用new創建一個對象，它使用反射爲您創建GraphicsEnvironment。在這種情況下，調用newInstance()會創建一個您正在使用的類的新實例，除非您運行的是無頭的;在這一點上它是直接用new關鍵字創建的。