小编Ran*_* Ji的帖子

System.gc();
System.out.println((Runtime.getRuntime().totalMemory()-Runtime.getRuntime().freeMemory())/1024/1024);// print 1M

Map map=new HashMap();
for(int i=0;i<100000;i++){
    map.put("key"+i,"i");
}
System.gc();
System.out.println((Runtime.getRuntime().totalMemory()-Runtime.getRuntime().freeMemory())/1024/1024); //print 10M

map.clear();
System.gc();
System.out.println((Runtime.getRuntime().totalMemory()-Runtime.getRuntime().freeMemory()));//print less than 1M

看来调用clear方法时内存会减少。然而，从其他答案来看，该clear方法似乎从未缩小HashMap. 那么为什么内存会减少呢？

有三个viewController，MainViewController ViewControllerB和ViewControllerC。 MainViewController将在应用程序启动时加载。

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {

    // Override point for customization after application launch.

    self.window = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen] bounds]];

    MainViewController * main = [[MainViewController alloc]init];
    UINavigationController * navigationController = [[UINavigationController alloc]initWithRootViewController:main];
    self.window.rootViewController = navigationController;
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    [[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleLightContent];
    return YES;       
}

并且有一个button关于MainViewController，现在ViewControllerB，

UIViewController *rootViewController = [[UIApplication sharedApplication].keyWindow rootViewController];
ViewControllerB * vcb=[[ViewControllerB alloc] init];
[rootViewController presentViewController:vcb animated:YES completion:nil];

在之后 …