if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {    
           //  大于等于24即为7.0及以上执行内容  
       } else {  
           //  低于24即为7.0以下执行内容  
       }

如果没有用到Build.VERSION_CODES.N，版本比较低，可以直接用数字代替，例如Build.VERSION_CODES.N对应24，其他的可以在Build文件中查找到，以下是对应的版本号

/** 
* October 2008: The original, first, version of Android.  Yay! 
*/  
public static final int BASE = 1;  

/** 
* February 2009: First Android update, officially called 1.1. 
*/  
public static final int BASE_1_1 = 2;  

/** 
* May 2009: Android 1.5. 
*/  
public static final int CUPCAKE = 3;  

/** 
* September 2009: Android 1.6. 
*/  
public static final int DONUT = 4;  

/** 
* November 2009: Android 2.0 
*/  
public static final int ECLAIR = 5;  

/** 
* December 2009: Android 2.0.1 
*/  
public static final int ECLAIR_0_1 = 6;  

/** 
* January 2010: Android 2.1 
*/  
public static final int ECLAIR_MR1 = 7;  

/** 
* June 2010: Android 2.2 
*/  
public static final int FROYO = 8;  

/** 
* November 2010: Android 2.3 
* 
*/  
public static final int GINGERBREAD = 9;  

/** 
* February 2011: Android 2.3.3. 
*/  
public static final int GINGERBREAD_MR1 = 10;  

/** 
* February 2011: Android 3.0. 
* 
*/  
public static final int HONEYCOMB = 11;  

/** 
* May 2011: Android 3.1. 
*/  
public static final int HONEYCOMB_MR1 = 12;  

/** 
* June 2011: Android 3.2. 
* 
*/  
public static final int HONEYCOMB_MR2 = 13;  

/** 
* October 2011: Android 4.0. 
* 
*/  
public static final int ICE_CREAM_SANDWICH = 14;  

/** 
* December 2011: Android 4.0.3. 
*/  
public static final int ICE_CREAM_SANDWICH_MR1 = 15;  

/** 
* June 2012: Android 4.1. 
*/  
public static final int JELLY_BEAN = 16;  

/** 
* November 2012: Android 4.2, Moar jelly beans! 
*/  
public static final int JELLY_BEAN_MR1 = 17;  

/** 
* July 2013: Android 4.3, the revenge of the beans. 
*/  
public static final int JELLY_BEAN_MR2 = 18;  

/** 
* October 2013: Android 4.4, KitKat, another tasty treat. 
* 
*/  
public static final int KITKAT = 19;  

/** 
* June 2014: Android 4.4W. KitKat for watches, snacks on the run. 
* 
*/  
public static final int KITKAT_WATCH = 20;  

/** 
* Temporary until we completely switch to {@link #LOLLIPOP}. 
* @hide 
*/  
public static final int L = 21;  

/** 
* November 2014: Lollipop.  A flat one with beautiful shadows.  But still tasty. 
* 
*/  
public static final int LOLLIPOP = 21;  

/** 
* March 2015: Lollipop with an extra sugar coating on the outside! 
*/  
public static final int LOLLIPOP_MR1 = 22;  

/** 
* M is for Marshmallow! 
* 
*/  
public static final int M = 23;  

/** 
* N is for Nougat. 
*/  
public static final int N = 24;  

/** 
* N MR1: Nougat++. 
*/  
public static final int N_MR1 = 25;  

感谢http://blog.csdn.net/bluewindtalker/article/details/53997613