解讀java.lang.System

public final class System {    private System() {}     public final static PrintStream out = getConsoleOutput();    private static PrintStream getConsoleOutput() {        return new PrintStream(new SystemOutputStream());    }     public final static PrintStream err = out;    public static native long currentTimeMillis();     public static native void arraycopy(Object src, int src_position,                                        Object dst, int dst_position,                                        int length);     public static native int identityHashCode(Object x);     public static String getProperty(String key) {        if (key == null) {            throw new NullPointerException("key can't be null");        }        if (key.equals("")
 ) { throw new IllegalArgumentException("key can't be empty"); } return getProperty0(key); }  private native static String getProperty0(String key);  public static void exit(int status) { Runtime.getRuntime().exit(status); }  public static void gc() { Runtime.getRuntime().gc(); }}

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System.out應用SystemOutputStream。我們看看它是怎樣的：

public class SystemOutputStream extends OutputStream {    synchronized public void write(int c) throws IOException {        putchar((char) c);    }     private static native void putchar(char c);}

 可以看到跟C裡面的putchar是一樣的，實際上是調用C的putchar。System是靜態類。供給系統的方便方法。System沒有輸進流。留心手機並沒有把持台，像window下的DOS或者Linux的bash shell。沒有該方面的功效。當程序運行在手機上，調用System.out.println()是不會被顯示的。只僅僅調試的時候才有把持台。但該把持台只能輸出。