//此实例来自《java核心技术》卷一
 class MemoryDemo{ 
    public static void main(String args[]){ 
        Runtime r = Runtime.getRuntime(); 
        long mem1,mem2; 
        Integer someints[] = new Integer[1000]; 
        System.out.println("Total memory is ：" + r.totalMemory()); 
        mem1 = r.freeMemory(); 
        System.out.println("Initial free is : " + mem1); 
        r.gc(); 
        mem1 = r.freeMemory(); 
        System.out.println("Free memory after garbage collection : " + mem1); 
        //allocate integers 
        for(int i=0; i<1000; i++) someints[i] = new Integer(i);  
        mem2 = r.freeMemory(); 
        System.out.println("Free memory after allocation : " + mem2); 
        System.out.println("Memory used by allocation : " +(mem1-mem2));  
        //discard Intergers 
        for(int i=0; i<1000; i++) someints[i] = null; 
        r.gc(); //request garbage collection 
        mem2 = r.freeMemory(); 
        System.out.println("Free memory after collecting " + "discarded integers : " + mem2); 
    } 
}

Total memory is ：2031616
Initial free is : 1818488
Free memory after garbage collection : 1888808
Free memory after allocation : 1872224
Memory used by allocation : 16584
Free memory after collecting discarded integers : 1888808

//此实例来自《Java核心技术》卷一
class ExecDemo { 
    public static void main(String args[]){ 
        Runtime r = Runtime.getRuntime(); 
        Process p = null; 
        try{ 
            p = r.exec("notepad"); 
        } catch (Exception e) { 
            System.out.println("Error executing notepad."); 
        } 
    } 
}

//此实例来自《Java核心技术》卷一
class ExecDemoFini {
  public static void main(String args[]){
    Runtime r = Runtime.getRuntime();
    Process p = null;
    try{
      p = r.exec("notepad");
      p.waitFor();
    } catch (Exception e) {
      System.out.println("Error executing notepad.");
    }
    System.out.println("Notepad returned " + p.exitValue());
  }
}