some information about interface

diary/2021/2021_04.tex

@@ -218,12 +218,21 @@ https://developer.android.com/training/basics/firstapp
 
 Installed java 8 so i can compile the build.gradle file but for this wasn't enough time
 
+\def\day{\textit{April 15st Simon, 2021}}
+\def\weekday{\textit{Thursday}}
+\subsection*{\weekday, \day}
 
+I finally could compile the testapp to an apk file. 
 
+\def\day{\textit{April 19st, 2021}}
+\def\weekday{\textit{Thursday}}
+\subsection*{\weekday, \day}
+Interfaces specify what a class must do and not how. It is the blueprint of the class.
+An Interface is about capabilities like a Player may be an interface and any class implementing Player must be able to (or must implement) move(). So it specifies a set of methods that the class has to implement.
+If a class implements an interface and does not provide method bodies for all functions specified in the interface, then the class must be declared abstract.
+A Java library example is Comparator Interface. If a class implements this interface, then it can be used to sort a collection.
 
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+\includegraphics[width=.4\textwidth]{media/SyntactsInterface.png}