日志工具Log（android.util.Log），共有五個方法打印日志

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!");
    }
}

級別從低到高：

• Log.v() 最為瑣碎的，意義最小的日志信息，對應級別verbose
• Log.d() 打印調試信息，最常用，對應級別debug
• Log.i() 打印比較重要的數據，可以幫助分析用戶行為數據，對應級別info
• Log.w() 打印警告信息，此處有潛在危險，需要修復，對應級別warn
• Log.e() 打印程序中的錯誤信息，，必須修復，對應級別error

注1：tag 參數一般為當前的類名，msg想打印的具體信息

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

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

注2：System.out.println()，雖然在eclipse中輸入syso，按下代碼提示鍵就會出來，但在Android studio中不支持這種快捷方式。打印日志的的優點：可以添加過濾器、有等級區分、打印可控制、有打印時等。
注3：快捷輸入，如輸入logd，然后點擊tab鍵，就會補全。
在onCreate()外面輸入logt，點擊tab，就會以當前的類名作為值生成一個TAG常量

private static final String TAG = "MainActivity";

注4：添加過濾器，選擇顯示級別，搜索等等

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