嗨，我只是使用stringbuilder使用substring（int start，int end）获取字符串的某些部分。但是我遇到了这个错误

08-17 20:27:40.737: E/AndroidRuntime(15313): Caused by: java.lang.StringIndexOutOfBoundsException: length=3; regionStart=0; regionLength=4

08-17 20:27:40.737: E/AndroidRuntime(15313):    at java.lang.AbstractStringBuilder.startEndAndLength(AbstractStringBuilder.java:216)

08-17 20:27:40.737: E/AndroidRuntime(15313):    at java.lang.AbstractStringBuilder.substring(AbstractStringBuilder.java:625)

08-17 20:27:40.737: E/AndroidRuntime(15313):    at java.lang.StringBuilder.substring(StringBuilder.java:44)

08-17 20:27:40.737: E/AndroidRuntime(15313):    at com.example.john.Contact.getAllCallLogs(Contact.java:196)

08-17 20:27:40.737: E/AndroidRuntime(15313):    at com.example.john.Contact.onCreate(Contact.java:49)

08-17 20:27:40.737: E/AndroidRuntime(15313):    at android.app.Activity.performCreate(Activity.java:4479)

08-17 20:27:40.737: E/AndroidRuntime(15313):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1050)

08-17 20:27:40.737: E/AndroidRuntime(15313):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2041)

这就是我简单实现的方式：

StringBuilder str = new StringBuilder(phoneNumber);
          String fourth = str.substring(0, 4);

我只想获取我的phoneNumber字符串的前4个字符