日志工具Log（android.util.Log），共有五个方法打印日志

package android.util;

public final class Log {
    public static final int ASSERT = 7;
    public static final int DEBUG = 3;
    public static final int ERROR = 6;
    public static final int INFO = 4;
    public static final int VERBOSE = 2;
    public static final int WARN = 5;

    Log() {
        throw new RuntimeException("Stub!");
    }

    public static int v(String tag, String msg) {
        throw new RuntimeException("Stub!");
    }

    public static int v(String tag, String msg, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static int d(String tag, String msg) {
        throw new RuntimeException("Stub!");
    }

    public static int d(String tag, String msg, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static int i(String tag, String msg) {
        throw new RuntimeException("Stub!");
    }

    public static int i(String tag, String msg, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static int w(String tag, String msg) {
        throw new RuntimeException("Stub!");
    }

    public static int w(String tag, String msg, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static native boolean isLoggable(String var0, int var1);

    public static int w(String tag, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static int e(String tag, String msg) {
        throw new RuntimeException("Stub!");
    }

    public static int e(String tag, String msg, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static int wtf(String tag, String msg) {
        throw new RuntimeException("Stub!");
    }

    public static int wtf(String tag, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static int wtf(String tag, String msg, Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static String getStackTraceString(Throwable tr) {
        throw new RuntimeException("Stub!");
    }

    public static int println(int priority, String tag, String msg) {
        throw new RuntimeException("Stub!");
    }
}

级别从低到高：

• Log.v() 最为琐碎的，意义最小的日志信息，对应级别verbose
• Log.d() 打印调试信息，最常用，对应级别debug
• Log.i() 打印比较重要的数据，可以帮助分析用户行为数据，对应级别info
• Log.w() 打印警告信息，此处有潜在危险，需要修复，对应级别warn
• Log.e() 打印程序中的错误信息，，必须修复，对应级别error

注1：tag 参数一般为当前的类名，msg想打印的具体信息

 private String tag = MainActivity.class.getSimpleName();

    /**
    * Activity 被创建时
    */
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        Log.d(tag, "onCreate");
    }

注2：System.out.println()，虽然在eclipse中输入syso，按下代码提示键就会出来，但在Android studio中不支持这种快捷方式。打印日志的的优点：可以添加过滤器、有等级区分、打印可控制、有打印时等。
注3：快捷输入，如输入logd，然后点击tab键，就会补全。
在onCreate()外面输入logt，点击tab，就会以当前的类名作为值生成一个TAG常量

private static final String TAG = "MainActivity";

注4：添加过滤器，选择显示级别，搜索等等

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