Eclipse java compilation ?!

I found something strange conrens Eclipse. The point is, that Eclipse compile even invalid java code - and, somehow runnable. Take a look on these source files:

package test;

public interface IntA {
    void A();
    void B();
    void C();
    void D();
    void E();
}

package test;

public class ClsB implements IntA{

    public void A() {
        System.out.println("A");
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("Howdyyyy");
        IntA a = new ClsB(); 
        a.A();
        // a.B(); when uncommented, class doesn't compile
    }

}

All right. This class would be compilable even when is marked as "abstract". So, any attempt to compile with javac results to:

K:\UFO_G3\workspace\test>c:\jdk5\bin\javac.exe test/*.java
test/ClsB.java:3: test.ClsB is not abstract and does not override abstract metho
d E() in test.IntA
public class ClsB implements IntA{
       ^
1 error

as expected. But when in Eclipse, everything is OK but you can't even run such class with this result:

Nebylo možno načíst třídu prostředí Java: java.lang.UnsupportedClassVersionError: test/ClsB (Unsupported major.minor version 49.0)
And I'm asking: why is possible to compile an invalid class ? Why Eclipse using some internal compiler with a non-standard results ? Perhaps that's the reason why I'm using eclipse only as an editor and compiling classes with the Ant ...