在使用actionbar的时候可能会出现这样的报错
This Activity already has an action bar supplied by the window decor. Do not request Window.FEATURE_ACTION_BAR and set android:windowActionBar to false in your theme to use a Toolbar instead.
刚开始看到这个报错会觉得比较奇怪，但是仔细分析一下就能发现问题的原因了
首先在源码中（使用Android xref）找到报异常的位置，最后在activity.java找到如下代码

public void setActionBar(@Nullable Toolbar toolbar) {
        final ActionBar ab = getActionBar();
        if (ab instanceof WindowDecorActionBar) {
            throw new IllegalStateException("This Activity already has an action bar supplied " +
                    "by the window decor. Do not request Window.FEATURE_ACTION_BAR and set " +
                    "android:windowActionBar to false in your theme to use a Toolbar instead.");
        }

        // If we reach here then we're setting a new action bar
        // First clear out the MenuInflater to make sure that it is valid for the new Action Bar
        mMenuInflater = null;

        // If we have an action bar currently, destroy it
        if (ab != null) {
            ab.onDestroy();
        }

        if (toolbar != null) {
            final ToolbarActionBar tbab = new ToolbarActionBar(toolbar, getTitle(), this);
            mActionBar = tbab;
            mWindow.setCallback(tbab.getWrappedWindowCallback());
        } else {
            mActionBar = null;
            // Re-set the original window callback since we may have already set a Toolbar wrapper
            mWindow.setCallback(this);
        }

        invalidateOptionsMenu();
    }

能够走到这段代码，说明应用是使用toolbar来代替actionbar的
首先会获取当前的actionbar对象ab，如果已经设置toolbar，则类型为Toolbar，否则为默认的WindowDecorActionBar，然后进行类型判断
如果ab的类型为WindowDecorActionBar，表明已经设置过默认actionbar了，将WindowDecorActionBar替换成Toolbar是不被允许的，所以抛出异常

出现这种情况一般可能由以下几种原因

• 当前theme的parent含有属性<item name="android:windowActionBar">true</item>
• 当前activity的父类可能执行了对actionbar的操作，例如getActionBar，setDisplayHomeAsUpEnabled
  知道问题的原因后，解决也就很容易了