先看一段代碼：
import java.io.IOException;
public class RuntimeTest {
public static void main(String[] args) throws IOException, InterruptedException{
Process p=Runtime.getRuntime().exec("notepad.exe");
p.waitFor();
System.out.println("end!");
}查看文檔，發現Process類的waitFor()方法是一個abstract方法。可是爲什麼可以直接調用啊？？？
恩，直覺上想，應該是下面這行代碼，實際返回的是Process類的一個子類，在子類中重寫了waitFor()方法。
Runtime.getRuntime().exec("notepad.exe");Runtime的源碼，重載了好幾個exec方法，但所有的exec方法都是調用下面這個方法來實現的。
public Process exec(String[] cmdarray, String[] envp, File dir)
throws IOException {
return new ProcessBuilder(cmdarray)
.environment(envp)
.directory(dir)
.start();
}那我們再來查看ProcessBuilder的start方法源碼：
public Process start() throws IOException {
// Must convert to array first -- a malicious user-supplied
// list might try to circumvent the security check.
String[] cmdarray = command.toArray(new String[command.size()]);
cmdarray = cmdarray.clone();
for (String arg : cmdarray)
if (arg == null)
throw new NullPointerException();
// Throws IndexOutOfBoundsException if command is empty
String prog = cmdarray[0];
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkExec(prog);
}
String dir = directory == null ? null : directory.toString();
try {
return ProcessImpl.start(cmdarray,
environment,
dir,
redirects,
redirectErrorStream);
} catch (IOException | IllegalArgumentException e) {
String exceptionInfo = ": " + e.getMessage();
Throwable cause = e;
if ((e instanceof IOException) && security != null) {
// Can not disclose the fail reason for read-protected files.
try {
security.checkRead(prog);
} catch (AccessControlException ace) {
exceptionInfo = "";
cause = ace;
}
}
// It's much easier for us to create a high-quality error
// message than the low-level C code which found the problem.
throw new IOException(
"Cannot run program \"" + prog + "\""
+ (dir == null ? "" : " (in directory \"" + dir + "\")")
+ exceptionInfo,
cause);
}
}package java.lang;
import java.io.IOException;
import java.io.File;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.FileDescriptor;
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.lang.ProcessBuilder.Redirect;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
/* This class is for the exclusive use of ProcessBuilder.start() to
* create new processes.
*
* @author Martin Buchholz
* @since 1.5
*/
final class ProcessImpl extends Process {
<pre name="code" class="java">/*
*中間部分代碼省略
*//*
*中間部分代碼省略
*/
public int waitFor() throws InterruptedException { waitForInterruptibly(handle); if (Thread.interrupted()) throw new InterruptedException(); return exitValue(); } private static native void waitForInterruptibly(long handle);} 可以看出，ProcessImpl類是Process類的一個子類，start方法最後返回了一個ProcessImpl對象。而且ProcessImpl重寫了waitFor方法。
